So far all the replies have focused on the efficacy element. But safety is required as well.
Safety cannot be demonstrated until 81 days (60 days after dose 2) after the 15,000th participant enrolled in the trial. Based on enrollment press releases, that occurs on April 19th +/- 1 day.
As such, efficacy data to support application could have been available weeks ago, but crossover cannot be initiated until after that safety milestone.
There was some uncertainty as to what the FDA would allow from the UK trial, if anything, but it seems highly likely to me that they decided to not allow the UK to contribute to the 60 day safety window. The timing of crossover is too conspicuous.