Presumably everyone commenting has also seen his comment about removing the censorship to get access to all the stuff deemed too dodgy for it to permitted in the UK
The full adult version like all other visual novels are fully legal and permitted in the UK.
It's just that VN's on Steam are normally censored and require patches that are provided elsewhere by the developers for a extra price that allow you to patch the game on Steam.
It's kinda scrummy practice to do because there is no need to do it, but you forced to pay for the the game and then patch or if no patch is provided, you have to pay for the game on Steam and then the Full version on a other website and just copy and paste the files over to the Steam version just to gain access to like 1% of the game.
OR... you can just avoid Steam and just buy the full version elsewhere and get a hard disk copy or a download link from a other retailer.
I just wanted to certify that.