0 competition and regulatory capture so nobody new can come in
But most people in the UK sign-up to the Big Three ( BT Retail, Virgin, TalkTalk ) due to their intense advertising and bundle 'deals'. So those three consider themselves the market and compete with one another for that pool of customers. Naturally they make leaving difficult because they have spent so much on acquisition.
The other 20% of customers shop around based on individual requirements. I've been with my small, efficient ISP for 12 years and have no reason to change.