I reckon the virus would spread further and faster than has been suggested.
Some people, at least, are carriers and would carry it with them as they flee in vehicles, boats etc. People travelling with drugged up or restrained infected would spread it faster than walking pace. Until people get close Mailer was pretty docile when chained up.
Plus we don’t really know how long the virus can last in the open, but it must be sometime given how Frank was infected. If it can survive on a traffic poll for days, it can probably survive in blood on someone’s car or shirt. Worse, it might survive inside animals, and you’d get people in Europe infected by eating fish that have consumed some of the dead.
And just in terms of population movement. If everyone is fleeing from the cities and motorways, then low density areas are going to get flooded. In the first film I think Henry says the countryside is “teeming with infected”.
This. You don't need your normal infected to actually travel to the outer Hebrides at all. All you need is a normal guy on a boat traveling there with some infected blood on his shirt. When he gets there he go's to wipe the sweat off his forehead. Then he get's the tiniest bit of that infected blood mixed in with the sweat on his forehead that then drips down into his eyes.
And boom now you've got an infected in what was otherwise "Virgin Field" territory. You also risk incidents of deluded desperate people managing to tie up or drug infected family members and try to transport them to otherwise uninfected regions.
And it is theoretically possible for infected to be transported by ship or boat overwater in uncontrolled vessels. Just have them get locked up or trapped in say the pilot house of a fishing vessel or one of the rooms of a larger ship and have the ship washed by the tide onto shore of some formerly safe region. For larger modern vessels I believe there's an equivalent to an "Auto pilot" on a plane. The ships course and destination is laid in and the ship can mostly travel there on it's own. So have a vessel with one infected on it and the rest of it's complement be uninfected at first. The uninfected crew put in the coordinates of their destination and then succumb as the outbreak explodes.
You could also have say one of the earlier scenarios (the bit of infected blood on a shirt mixing with sweat and dripping into the eyes) happen on say a commercial jet. Have the infection rage in the main cabin but the pilots remain safe and uninfected behind reinforced cabin door in the cockpit. Then have them try and land spilling their infected cargo across the landing area.