You will find that the Merlin is about the same size as the Hughie or UH1 but I think it has four blades not two and they are on skids and more heavy,
A post that is usless to the point of being pointless based on how a merlin is about 3 times the mass of a UH-1, is on wheels and is considerably larger all round, not to mention things like disc loading likely rotor wash etc.... so almost exactly
not like a hughy in every respect.
Westlands should be able to provide advice , after all they build the thing. I'd be tempted to phone them up and ask (+44 1935 475222).
Some reading for you could include ICAO Annex 14 Volume 2 which although intended for licenced helicopter aerodromes includes a lot of guidance as to FATO (Final Approach/Take Off) clearances and required areas.
Also lighting and markings, surface materials etc.
The CAA in Gatwick should have a Civil Air Publication (CAP) covering similar UK requirements. (possibly 168 though that may just be fixed wing)
Speak with current UK Merlin operators - the RAF and RN and ask for their requirements.
I suspect the merlin requirement is because it is the largest helicopter likely to be entered into the competition for the UK's future SAR needs and as such a pad designed for a merlin should fit anything else.
Edited to add - Like the BHAB guide, consider also
FAA planning guide Ok its american but it has nice pictures that make things more obvious.