We got mechanism from Wall Bed King. We're very happy with it - seems robust, was well priced (as wall beds go), and they were very helpful.
You can buy cabinets from them too, or I did find somewhere else that makes them (will dig out the link if you're interested), or you can do as we did and have a carpenter build something around them. The latter might be better for you if you want top cabinets, but might also be more expensive.
Also a caveat on the recommendation - no one has actually slept on ours yet (stupid lock downs/Tier 3), but I don't anticipate any issues. It's really sturdy.
NextBed might be worth looking at too - they have a few UK suppliers, and seem very well reviewed.