As a club sec.I just have to take their word for it,with the consequences of being found out by DEFRA I would say that any serious pigeon racer would not take that chance as the birds could be tested at any time by a official when being transported.
Was told years ago by a real expert at racing darkness ybs to take them off 1 month before the first race, although I must admit I took mine off on the longest day last year and had a very good yb season,so its the 21st again this year for me.