Whilst I hesitate to agree with TC as he so often tends to forcibly overstate things, for once I thnk he's right. I have no knowledge of UK operations, but where I fly (often in sparsely-populated areas in mountains considerably higher than those in Snowdon) monitoring of 121.5 is totally impracticable. I have to monitor a company operations frequency on one VHF radio, an area common frequency on another VHF radio, a client frequency on my HF radio and another client frequency on my FM radio. If I have an emergency I rely on my transmissions being picked up on the radio I last managed to communicate on, or if need be, either by manual operation or crashing, the satellite picking up my signal on 406. If I survive I then use my Ascom satellite phone (considerably cheaper than installing a third VHF radio) to try and let my company know where I am so I can be rescued.
In reality, I would imagine that in most countries, most pilots would first put out a distress call on the frequency on which they last managed to make radio contact and 121.5 would only be used as a last resort.