He speaks with a pretty refined and educated variant of the British accent from the south of the country. He lived in Wales until he was 10 but there's no trace of a Welsh accent that I could hear. I would say he sounds like a very well-educated Londoner speaking carefully and politely in order to sound neutral and be easily understood. Some North Londoners speak like him, but not all.

Ken Follett is British and so is his accent. You can listen to him here: https://www.youtube.com/user/KenFollettAuthor