Ham - the back of the IMC form has a section where you claim "exemptions" from either 1) flight test, 2) written examination or, 3) all of the above.
"Check" option 3 - write in alongside if you wish "FAA Instrument Rating" - send to Gatwick before March 31st. (Done this MANY times)
TMG - just fill out the application - best of knowledge stuff - send off before March 31st with covering note claiming exemptions listed in LASORS.
If there is a better way - CAA will contact you directly.