I had my second dose in the UK after 5 weeks, by just going to a walk in centre and asking - apparently they had way too many doses they would be wasted otherwise, so they were happy to give it to anyone asking. Apparently the absolute NHS-mandated minimum is 3 weeks between doses, that's the one they can't disobey. But the 8 week minimum is much more flexible.
Looking at just the Cumulative data, ~79% of people in the UK that have taken the first dose have taken the second dose. Looks like it is practical. And that number doesn't account for people that haven't had their second appointment yet.