hi there I think it definitely looks like a Fairmile 'D'. A Fairmile 'B' has a round-shaped hull. The only thing, the keel looks as if it stops a little bit short on the bottom, but hey that is easily rectified. Plus, she has only has twin screws. This may just be a model builder's preference; she should have four screws.
Some of these craft were used by the Royal Air Force for air sea rescue work. I am not 100% sure, but, I do believe that one or two of these air sea rescue craft were only twin screwed. CDS123 is the man to say - yay or nay for that; however, I am sure I read somewhere that one of the things they did to increase range of these craft - while used by air sea rescue was, to remove the 2 inboard props and motors and install long range tanks in their place.
For further reference, you could look for the book written by John Lambert, Coastal Forces and also another book The Anatomy of the Ship The Fairmile 'D; motor torpedo boat, also by John Lambert
ISBN:0 84486 006.
Hope this is of some help.
aye
john e
bluebird