Quick Google shows Merlin having XT / SLX in stock. Sports Direct seem to have XT 12 speed in stock too. Leisure Lakes also have SLX in stock. Buy while you see it as it’ll be gone before long no doubt…..
FWIW when I built MrsP’s 1×12 bike I got the wrong mech but she wanted to give it a go so I tried fitting an old SLX 11sp mech in the hope that she’d get some shifting even if it wasn’t perfect. It worked fine, even in the biggest sprocket – so much so that I wasn’t in that much of a rush to replace it when the correct 12sp mech arrived.