Can't you order those in an online tire shop, have them shipped to the UK and have them installed by your local tire-mounting-guy
?
and why stick with the RFT? They give you a uncomfortable ride and the 150km that you can drive them when flat are useless when on a vacation trip (been there, done that...)
you might need a spare tire kit though...
or you can order them in Belgium, my tire dealer has those available (on order only of course...)
When passing through Germany and Austria, make sure you have those wintertires because the Polizei is on the lookout...