This is the one I have. It’s at the expensive end of the scale for a clone sight, but it’s available in the U.K. doesn’t have much parallax error and holds a rock solid zero even on a GBBR.
https://www.airsoftzone.co.uk/553-type-red-green-dot-sight-gp.html
If you are willing to chance ordering from abroad one of the guys ordered one of these. The one he got was very good, but you do take a bit of a chance ordering this way.
Vortex clone