Have only read that the aircraft was lost from radar, I would assume at low level this would happen between Welshpool and Bala. Upon reaching on a westerly track, Rhinog Fawr is is 700 metres AMSL, and flying west from Trawsfynydd at 229 metres AMSL, the terrain goes op to 366 metres AMSL, this is serious mountain territory. Valley is 10 metres AMSL, at with a reported clouse base of 300 ft at the time. Not sure if I agree Trawsfynydd was mistaken for the sea as someone pointed out is is quite small and does not has a coastal look.
Was the aircraft ever picked up on radar over the sea, and even if it ever made it over the sea, would anyone turn back into mountains, the sensible thing to do would have been to continue west and request VDF QDMs or radar headings from RAF valley once clear of the Phwelli penisulla divert into RAF valley below 300ft, ATC would probly able to advise when it was safe to turn right onto a northely heading.
What I do not understand as there seems a different thinking in some rotary wing pilots compared to fixed wing regading MSA and low level flying. I think this is based on the fact that some rotary wing pilots think in the back of their mind, "well if it gets bad, I can land on a field", that certainly would have been an option Welshpool. Bala and Trawsfynydd which would have had playing fields, and there is an abundance of B&B's and country hotels, welsh hospitality and lamb suppers. My second thought is that the second thought patterm may be, "if the weather gets bad, I'll do a 180 turn and fly back out of the valley. Interestingly rarely are of these options are available to a fixed wing pilot. Once you enter valley in less that optimal conditions you are committed. My other difficulty it that how rapidly mountain weather can change, in a variety of ways, I've seen rolling fog form below me while walkingon the fells in the lakes in less that twenty minutes.
The RAF Chinook accident, from Belfast to Inverness, which flew into the Mull of Kintyre with very important passengers, in poor weather, why had they not planned to climb to MSA upon poor weather? Likewise the rotary wing accident in London, where the aircraft flew into a crane on top of a tower block, the pilot is poor weather, and thinks 100ft clearance over a high building is acceptable, worse still he was using his mobile, for which I do not think the AAIB stated was a factor or was wrong.
While I do not want to single out helicopter pilots for CFIT in poor weather, I recall an fixed wing accident, two private pilot's flying from the midlands to Blackpool in a C152, while over Staffordshire or Cheshire, they inadvertantly entered IMC, they did not hold IMC ratings, while intially they were as I recall working Manchester, they apparently decided to head due west and descend over the sea. Two problems, they would need to stay south of the Liverpool and Manchester Zones, but north of Snowdonia, end result they impacted with Tryfan at about 2700 ft with fatal results.
At the time the CAA Flight Handling Test (GFT) required to demonsrate inadvent entry in to cloud, but not having failed to resume VFR, immediate climb to MSA, declare emergency and radar vectors to a descent in a safe (metwise) area. This was a failure by CAA standards.