I'm not in SE England - in Scotland at the minute, although lived in the US before.
My twins are in P1 (reception) and have been separated. They were together the last 2 years in pre-k (like pre-school/nursery) but we were in a private school in the US and it was full days, uniform, much more structure and teaching than you would get here in pre-school settings.
We made the decision before they were born to keep them together until they started formal school and then to separate them. Due to a lot of moves which weren't planned, inc an unexpected move back to the UK last summer just before they started p1, we did think about whether we were wrong to separate them, as it was a lot of changes in one go, but on reflection we decided our original plan was for the best.
There are 3 sets of twins in our current p1 year - 2 sets of identical girls (of which my twins are one set) and a boy/girl set. The boy/girl set are together and both sets of identicals have been separated. The other set have probably found it easier than my girls have as although they are close, they aren't as close as my two. I think all the moves we have done over the last few years have really strengthened the bond between my two and they really are best of friends. They miss each other a lot in school and regularly say they wish they could be together, but they are actually fine in school and are good at making friends separately. I do wonder how different their experience would have been if we'd stayed in Scotland the whole of the last 6 years instead of moving abroad and forcing them into lots of big, big changes. Having said that - while their experience abroad has definitely made them closer - I has also made them (and my other two DC) really good at adapting to new situations, not getting stressed about things, taking things in their stride, and getting to know new people.
The hardest thing at the minute is when one gets a birthday invitation and the other doesn't. It's a new one for us and has happened twice to the same twin (just luck and timing really - the other twin is just as popular and happy at school) and it's been hard on the one left out, although we would never expect anyone to invite both just because they are twins and we have explained it to the girls repeatedly. I suspect those kinds of things wouldn't happen as much if they were in the same class.
They still choose to play together and eat lunch together almost every day at school, but I guess the main positive is that other people see them more as individuals and they are referred to a lot less as "the twins" now.