It MAY be possible to buy-in on the day but Luton is an extremely popular event and so it may sell out before the starting gun.
Alternates are allowed for the first couple of levels but entry is not guaranteed.
I understand that you don"t play online but there is still no reason you cannot download the Betfair poker software and only use it for buying in to the Live events. In fact, I only use it for that (and APAT satellites).