In England you need to go to Gatwick but I advice you to go to Holland.
Why? Because alomost all tests are available there. We got 3 different AMC's
http://www.aeromed.nl/ (Holmes Wright) Schiphol
http://www.aviationmedical.com/ (Anomaloscope) Hoofddorp
http://www.alc-srv.com/ (Copy of the Beyne test, but different) Utrecht
I advice you to go to Hoofddorp First. 2nd Utrecht. 3rd Schiphol.
Good luck