Stornaway, Gibraltar (with care), Isle of Man, St Mawgan (is it still there?), Belfast.
Basically, be prepared to test whenever the conditions arise.
Edit: on reflection, Gibraltar may be too gusty. We flew at IOM in gusty conditions and it was a good test of inappropriate windshear warnings.
Use Prestwick as a base and have the flexibility to cover the west of the UK; add Liverpool, Warton (PPO?), Leeds.