Keep in mind that despite the 'popularity' of certain aspects of Science Fiction, (I'm thinking things like Dan Dare here for example,
https://en.wikipedia.org/wiki/Dan_Dare) what was really 'popular' wasn't space travel but the hope for the future being just as 'good' as the past if not better even though real world events were making that seem less and less likely as time went on. Megaroc never went anywhere because at the time nobody really could imagine where the money to try something like that might come from in the public budget. It didn't help that Britain didn't get any V2's to play with either and had to depend on American data. And mind you none of that looked too good and the American's had and were using the damn Germans that designed the thing!
But let's say they manage to get a manned Megaroc into 'space', what is the world reaction? Likely there's a LOT of consternation and upset in both Washington and Moscow, (at least behind closed doors) but it would depend on WHEN exactly this happens as both sides are not ramping up 'space' efforts but ICBM efforts and depending on what the basic Megaroc can do THAT more than anything will be they key factor. There will likely be a lot of support and congratulations toward the UK and Commonwealth, especially if Collier's "Man Will Conquer Space Soon!" series has already come out. (The public just like OTL will have been 'primed' for such a thing but there will be a lot of "horror" and "surprise" that it wasn't America that did it!
) There would probably be less of a 'panic' though as England was seen as a near-technical peer to the US though that would increase pressure on the US to 'do' something to counter it.
If it's early enough Stalin might die a bit more publicly but the key point here is more WILL there be any "space race" as we know it in such a time-line?
Consider that such an early effort will make the Soviet military even less inclined to allow "stunts" (which is how they viewed Sputnik and the Cosmonauts) rather than "practical" efforts. Sure the US and USSR may aim to put a man into orbit first but more likely both will instead aim to put a satellite into orbit before the Commonwealth does. Now a good question here is if someone else gets a few of the more important 'firsts' (Commonwealth has the first "man-in-space" followed by the US say with the first satellite) then does the USSR even "play" at a space race they are not highly likely to win much from?
While the USSR gained a lot of international notoriety and prestige for having so many 'firsts' in OTL in truth their entire program was marginal at best and they took a lot more risks to get where they did than the US did. And at no time was the political or military support for anything beyond ICBM's and military satellites very deep
If they put a man into space and then don't follow up that's going to cause some issues. Sure satellites are great to have but if you START with a human being there will be certain expectations that I'm not sure the Commonwealth can afford to pursue and yet they can't ignore them either. What does the Commonwealth NOT buy to pay for Megaroc and the follow-on work that they did buy OTL?
Randy