Suprised that no-ones yet mentioned the sting in the tail with regards Euro membership: mileage earning rates in premium cabins. With UK based memership you get 2 x in Club and 3 x in First (and even 4 x in Concorde) but on the continent only 1.25 x for J and 1.5 x for F/R.
Add the odd promo or two and you can *really* build up the mileage quickly in the UK program for those F and R awards

(and if you're really smart, you do this whilst only paying the euro-fare levels

)
dnw