Steve Walker wrote:

I can't remember the number unfortunately, but Loctite used to do a
white thread sealant for pipework - we used to use it at work where PTFE
tape was banned. You used to put it on the male thread, screw it into
position and allow to set. It set rock hard, although it was (just)
possible to unscrew it if necessary.

I've seen a liquid PTFE sealer in Wickes

https://wickes.co.uk/slic-tite/p/227184