I thought that they were not permitted in the UK due to the way our plumbing is usually installed and the risk of ‘backwash’ in to the system?
The UK seems to have this massive paranoia about some freak situation where a shower hose is left in a bath or toilet with the tap open at the same moment some bizarre incident happens that causes the entire water system to go into reverse and the main in the street to suck water back into the water main from your house. Has it ever actually happened? If it did wouldn’t the future forward pressure sufficiently clean all the pipes again?
Anyway – to make it compliant you just need to fit a double check valve – the same as you should on any outside tap.
Also, you need to have an inline shut off – the gun trigger and hose won’t cope with the slight pressurisation/depressurisation from other taps in the house being used. You either have a tap, or much better, like the one I linked to, a shut off valve built into the wall bracket that shuts the water off when you put the thing in it’s ‘holster’.
Lastly, ideally you want warm water – i used one of the temp limiters that you’re supposed to fit on bath taps now and turned it down and it runs at the same pre-mixed temp as our hand wash sink.