@Ong use www.whattomine.com and it will tell you the relative payouts from mining . instead of buying £20 a month of crypto , you'd pay the leccy bill instead and payoff the card purchase. dont rely on the etherium mining profits though as eip1559 could kill off etherium mining but you'd then swap to some of the other coins.
you'll have to adjust for the electric price in the one box . im on .19$/kwh . you may be on higher especially if its a prepayment meter