Are you aware of the series of publications "The history of ROYAL AIR FORCE MARINE CRAFT 1918 - 1986" ?
Author G D Pilborough MIRT.
Appendix 1b covers the 200 class and contains a copy of Air Ministry drawing E6/515, general arrangement for the 37'6'' RAF GENERAL SERVICE LAUNCH.
Long out of print, but isbn no O 9510266 3 1. Pub Canimpex Publishing 1986, your local library should be able to obtain a copy for you.
My copies are subscription no 87 of 100. So precious that I'm afraid I will not loan them out.
However my understanding of copyright law is that if you make a copy for your own use that is legal, so should you travel to Yorkshire you will be welcome to use my copier.
Good luck with the project, I am waiting for a reply about the moulded hull and deck.
Cheers
Jim