A similar subject was aired on the Flyer forum a while back. Taking and regurgitating one post probably explains why RNAV IAPs haven't been widely adopted in the UK, yet:
Backs of fag packets are not a UK CAA approved design tool.
The protection areas applicable to IFPs (and they are different for each type of IFP - but someone, somewhere, in ICAO did a LOT of clever mathematics to work out what those criteria might be) take into account many variables - including the concept, for example, of 'spiral winds' in which the nominal track of the aircraft is assumed to be adversely affected by the 'worst case' winds applicable to the calculated altitude throughout the turn. Consequently, the areas to be examined for obstacles would surprise you; they are far larger than you would imagine. Unfortunately, hard past experience has shown that the protection areas are not unduly conservative.
However, that is far from the whole story; its the allied: satellite coverage prediction report, increased (new?) CAP 232 survey requirements, safety case, ACP (that's the 'killer'), flight validation database production, flight validation plan, flight validation itself, etc that rack the price up. However, if you want your IAP to be Approved by SARG for promulgation in the AIP (discrete IAPs are no longer allowed) then that's what you have to do. And if you are a GA aerodrome with AFISOs, then you can include the CAP 1122 process costs as well!
The OP's post is probably a shining example of the old adage "a little knowledge can be dangerous."