Once Manchester and Birmingham have a few A380 diverts and Gatwick is pushed on capacity then I believe one of BA's alternates is Madrid.
I don't think it's a big issue, there are well over 200 approved diversion airfields for the big whale.
Manston, Doncaster, Stansted, Glasgow, Dublin, Shannon and Brussels are all compatible and approved by Airbus as A380 alternatives and which are closer than Madrid, for example. BA have already used Shannon ( G-XLEA ).
http://com.airbus-fenice.customers.a...ty_network.pdf