How long are grandparents supposed to wait until they move on with their lives? Until their own kids have decided not to produce any more children of their own? Until the last grandchild is all grown up?
And, of course, if the son decides to move himself because of job or schools or whatever that'd probably be fine.
As for the London based son. Easy answer for him, move back to Cardiff if he wants to see his old mates. He chose to move away and 'abandon' them. If they were that important he may have given it more thought. And there is a motorway and a train service that runs between London and Cardiff. Last time I looked there were hotels too.
DS knows that, once he's settle with his own place, we'll be on the move. We live where we do because it's a nice place, it's near his school and his friends. But there are few transport links, we don't want to have to be driving when we're old just because we have no choice. So we'll be looking for somewhere with better links.
DS is fine with that but, to be honest, he doesn't have a choice. It's not up to him. He'll move away without a thought about the friends he leaves behind if it suits him, just as he should. And so will we. Just as we should.
They'll get over it and, if they don't, that's their problem. Why would any child be happy to see their mum or dad unhappy just to keep the old family home they, themselves, have moved away from?