With the new legislation (since September last year I think), you don"t have to be seated at the start for poker tournaments in the UK - and that goes for APAT tournaments as well.
So you can turn up late, and you"ll be able to take your seat and your chips (your stack will have been blinded away as if you were seated and folding every hand).
It"s best to be there, and be seated before the start so that you can hear all the announcements (such as when the breaks will be, etc.) and also so you can settle in and get your mind ready for the poker.
So if you"re late for whatever reason, you can still play. If you aren"t going to be able to make it though you should contact APAT ASAP, otherwise you"ll be entered into the tournament, it"ll cost you your £75 entry, and you"ll be denying someone else a chance to play.