The longest platforms currently in public use in the UK▸ are some 792m in length.
I was expecting someone to ask "Huh?" with this post.
Folkestone Eurotunnel Terminal.
That's noticeably longer than Gloucester's which I thought was the longest although I gather Colchester also was in the contest.
I think it's inevitable you'll be talking to your inner Joad at some point on this question. What
do you mean by platform, and
which length matters?
If the length is what's usable by passengers to board a train, then the Folkestone Shuttle facility has obvious problems. It's only been built for and is only used by vehicles, so for a start you should exclude the ends past the ramps that vehicles can't get to. And probably not even then.
Colchester is an absolute cheat. It's not one platform, it's a through platform and a bay end to end. So it fails the "could you put one train that long in it" test.
That leaves Gloucester, Cambridge, and Edinburgh (Waverley). All have one or more through platforms with a full crossover in the middle so each half can be used separately. Only Gloucester has a length quoted by
NR» for both platforms used as one, but I don't believe that number as it equals the sum of the two bits used separately. The other two are only listed as two platforms with a length each.
Since platform edges have now been rebuilt to new standards, and are not considered as usable without coping and yellow lines (and knobbly strips), this overall length of that part can be measured as the overall length. NR's operational length will be less, as a result of signalling rules. On the basis of the total fit for passenger use, Gloucester wins with at least 595 m, followed by Edinburgh with 546 m and Cambridge with 538 m. But of course other definitions are available, if you prefer.