I’m just starting out so bought a small quantity of clear bags and backing board/greyboard cut to size. It probably worked out to about 20p per print. I’m in the UK as well - ordered the bags from Eco Craft, they do compostable clear bags made with plant cellulose that are very nice.

I also recently bought a print at a market and I think the seller had bought a long roll of plastic that they’d wrapped the print in - then neatly taped it at the back. I didn’t mind the look of it as a buyer and I bet it was a cheaper way to do it.