This definitely looks to me like (as other posters have said) a homebuilt conversion of a MTB hull, rather than a model built from a plan. (The hull in the photo looks like a commercial fibreglass or plastic moulding to me, but I could be wrong). The hull form looks to me more like a short Vosper-type MTB than a Fairmile.
The links posted by Frank should be a good source of inspiration for completing the boat - a lot of WW2 MTBs and RAF boats were converted to motor yachts, passenger vessels etc. post-war. Some of the ex-RCN Fairmile conversions in the first link (BMPT forum) look very similar in style to yours.