My understanding of how the system works is that an exam centre has to be open and available to all, but it will no doubt come at a cost if you are not with Naples. Afterall there are the invigilators and venues etc to pay. Give the CAA in the UK a call or email them and see what the story is. Also be aware that the exams might not run every month as they could be linked to when Naples are running their modules so you might have to work in with that. No doubt it would still work out cheaper than flying to Gatwick though.