Forget all about the Telephone Preference Service. It is a paper tiger and next to useless. UK firms will routinely ignore it and foreign firms don't even know of its existence.
The only sure way of dealing with this is to take the phone off the hook, and leave it off. That is what I have been doing for 2 years now. I have told everybody that I know personally, that if they wish to speak to me, they will need to use my Mobile. They can call or text me. If its important, I can always put the phone back on the hook and call them back.
It did take a while for people to cotton on, and there are still a few people that baulk at paying the extra to call my Mobile but its the only way that I can have some peace.
Sometimes I will use the phone myself but forget to take it off the hook afterwards, and within minutes the nuisance calls start again. Its a modern plague !