You will need to provide the CAA with copies of the EASA ATO, the examiners medical and licence and a course completion certificate - the EASA ATO should provide all that for you along with the skills test form. if you use a simulator you will need to provide proof of certification for that as well.
Gatwick are not good at processing paperwork, electronic or otherwise - be prepared for a wait of a month or so once you submit your paperwork to get your licence back with your new type rating on.