You could contact the buyer, tell them of the error and cancel the transaction as item no longer available then relist it.
Make sure when you relist, you set formal options to restrict buyers to UK.
I've used the GSP several times and had no problems.
To be honest, no matter where you sell it to, via ebay you'll have the same problems with funds withheld until you have a few recent big sales so probably best to be patient and maybe put a deposit down on watch from here.