Zettle seems to be the most popular. I was 50/50 between that and PayPal, went with PayPal in the end, it was pretty handy to have the money appear instantly in my PP account! However their readers are shite, another one has just crapped out on me. I phoned PP and they aren’t making them any more (obviously acknowledging this 🤣). They have bought Zettle so the guy just told me to get one of them.
Haven’t got one yet so can’t really give you the ins and outs, but if it works broadly similar to PayPal’s offering there’s no contract or monthly/annual fee, they just take a percentage of the transaction so perfect for occasional use.
EDIT: looks like the money goes to your (normal) account in 1-2 days, no PP account option anymore. Fee is 1.75% on contactless or 1% if (I think) they pay via PayPal through a QR code link.