Wasn't it discovered a few days later that the virus is fast-transforming one? Of course NHS and WHO personnel where baffled but then when it was established that it could transformer victims in 12 seconds, the international health community would know what to do. Unfortunately came the asymptomatic carriers.
Just the first week alone the world would face a second depression worst than 1929 or even OTL's 2008.
I'm gonna have to agree with you there might be some form of riots or hate attacks on British citizens just like what happened to Chinese or East Asian people with COVID-19. We might even see a forced deportation of such.
True the fact that the virus transfers will be found relatively quick. But without the research from the original lab there will still be a lot of doubt. Especially about possible long term versions. Someone will theorize quick that while the virus we know does as in TL rapidly infect those bitten or espoused to infected fluids in the right place. But others will fear that besides that version there is a longer term airborne version. Basically that someone can become infected just by nearing an infected individual but that the airborne version is slow acting and six months to a year later they will instantly transform into the infected with all that entails. Hence many countries will arrest Brits who were in Britain within even a few months of the outbreak and forcibly quarantine them in military facilities and high security prisons.
There will undoubtedly be hate attacks by mobs and individuals against British refugees or expats who were abroad at the time. Probably more lynchings then beatings unfortunately. Countries not even worried about a potential long term airborne version might quite likely arrest and at least temporarily imprison Brits just to prevent them from being ripped apart by terrified angry lynch mobs. This might very well extent in certain nations to Non Brits who have accent's that would be seen as some as being similar to the various British accents (Australians, New Salamanders, perhaps the Irish, and even people who aren't Brits and never have been but learned English with a British accent). You'll see in a lot of countries people changing their names from stereotypical British names to less British names.
The question as to forced deportation is where? Who would really accept them? We might see Brits living outside of the Overseas possessions be forcibly deported to the British possessions. If Ireland (both the Republic and NI) avoid infection we might see most of those outside of the possessions be deported to NI or the possessions. Quite possible that those possessions get a lighter quarantine/containment.
As to the British forces abroad there are two main options as I can see them
A) They're allowed to return to the UK bypassing the quarantine. These will be on "One Way Flights/voyages" with planes or ships loaded with soldiers and supplies allowed to bypass the Quarantine under the explicit terms that the planes and ships aren't going to be able to leave again anytime soon. For things like cargo aircraft and combat aircraft I could see them being allowed to base out of say Ireland (assuming it's not infected), or France, Norway, some of the other offshore islands (if they remain uninfected), the Benelux, or perhaps Iceland. As long as they're doing airstrikes or air dropping supplies (Weapons, munitions, fuel bladders, spare parts, food, medical supplies, and the like) without actually landing in Britain (even in say a safe zone) they might be allowed to continue to operate to help Britain as well as they can.
B) Some or all of the Servicemen are refused to be allowed to return to Britain in time. If this occurs you might well very see some significant mutinies, large numbers of suicides, and numbers of mental breakdowns as servicemen can't deal with the guilt of not being able to help their countries and families (besides those military dependents located at one of the bases located in a "Safe Posting" like Germany.). In the medium/long term you might see well that the British units abroad become sort of "mercenary units" like the various old scale military units formed from exiles of countries that no longer exist (Irish and Scottish Regiments in continental Europe in the 1600s, Kwarzimen units after the fall of their empire, Free Polish/French/Dutch/Czech units during WW2, Hessian units during the 1700s, the Tibetan units maintained by India) and offer their services abroad. While most countries during the post infection Great Depression 2.0 will not be interested in accepting large numbers of foreign refugee's accepting organized military units that are already fully trained, experienced in combat operations and fighting together, many of them combat veterans, and at least at first fully equipped with virtually all the gear they need. You could see either most or all of the Exile units going to a single country or being divided between different countries (Say the Gurkha regiments going to India, Singapore, and Brunei) with many most likely going to Commonwealth countries that offer good terms like say Australia, New Zealand, perhaps Canada (though I figure many of the exiles will have a massive grudge about countries that were large parts of the Quarantine and the burning of Britain and Canada will much like the US probably heavily participate in the less pleasant bits of the containment such as burning and guessing every major British city and refusing to let starving refugee's leave Britain.
If Ireland (and Northern Ireland) survives intact then it seems quite possible that a new British government would be formed there. In which case the majority of British military units abroad would probably go with the NI govern. Ironically in the short term considering the military weakness of the Republic of Ireland quite probably means that the Irish (begrudgingly) actually request the Brits deploy some of the redeployed British Army/RN/RAF units to the Republic in order to keep themselves from getting overrun. Also seems quite possible the Republic will quite likely actually subsidize the garrisoning of British troops in Ireland.
It seems possible that even with the terrible worldwide economy that other countries (Even ones that don't directly contribute troops, gear, SAMs, guns, combat aircraft, ships, subs and the like) might contribute financially and contribute supplies to help keep the Irish/NI defenses going as well as fortifying a large chunk of fortifying the Northern French coast as well as bits of the coast of the benelux and the like other foreign countries with coasts right near the coast of Britain.
Quite likely the French unfortunately have to evacuate/evict pretty much all civilians living along a massive chunk of the French Northern Coast and declare that zone up to a certain number of miles inland to be a military zone. Reuse old WW2 and earlier fortifications and construct new ones along with networks of radars, SAMs, and AAA, airfields, artillery batteries, and the like. Heavily man that fortified zone. Along the shoreline do frequent and constant shore patrols either on foot or motorized. Even without actual living infected potentially reaching shore (Say on a small boats that they became trapped on or attempted to flee to France while alive and then became infected or the like) their will be a metric fuckton of dead infected or potentially infected corpses washing ashore in France (Say those who ran off or jumped off cliffs, those on boats and ships that got sunk by the quarantine patrol, or those poor desperate bastards who tried to swim across). The shore patrols will then radio in what they found, note the location, and most likely burn the corpses on site with thermite or napalm and then suck up all of the sand or dirt around the corpse/remains to be further dissolved or incinerated.
Too much risk of a infected corpse washing ashore say near Calais and having a poor dumb kid try and poke it with a stick and accidentally infecting themselves.
By the time of 28 Weeks late Northern France would probably be single region on Earth that's most heavily defended by a extensive Air defense Network composed of all sorts of mobile and stationary guns, Radars, and SAM and in the air tethered aerostats, fighters/interceptors, and AWACs providing constant coverage.
By the time that helicopter at the end of 28 Weeks later had reached the middle of the English channel without explicit orders it would have been downed by more Dakka then anything else on Earth.