Maybe try video games? You'll be spoilt for choices. Technically you may still need to pay for the game(s) if it's not free-to-play, but it's basically a one-time purchase. It's considered one of the cheapest hobbies one can have if you look solely at the cost-per-hour value it provides. Just avoid gacha or subscription-based games if you don't want to fall into the recurring payment trap. 

Or try UK licensed betting sites that provide sign-up offers where you get free bets or spins (just google for comparison sites, like https://www.ukbetoffers.co.uk or https://www.bestbetingsites.co.uk), and play only when you are offered free bets.