Just because you have capacity for more fuel doesn't mean that you always need to top up all the tanks. I'd imagine for interceptor style missions the wing tanks would be left empty. And for aircraft entirely dedicated to the interceptor mission, like during the BoB, I imagine airfield mechanics could even remove the wing tanks for a small weight reduction. Or even not bothering to install them at the factory, such that wing tank kits could be manufactured later and delivered to the squadrons as the RAF adopts a more offensive posture post-BoB.
Further, with my suggestion above to move the radiators to the centerline, the roll rate should slightly improve compared to the historical Spitfire (as long as the wing tanks are empty, of course).