Legal? That's question for your lawyers in the UK.
Worthwhile? Well, there are several successful sites out there like that o there is a market for it. I certainly don't understand the appeal myself, but it does appeal to people and can be used to make money.
Now, as to yuor contention that a solo girl site might have been minimal money that is NOT the case. Sure some niches make more tan others but a good marketer can make good money for whatever niche they choose to do if done right.
One of the programs that impresses the hell out of me is
http://www.ftvcash.com Single girl sites plus a megasite and it's primarily promoted through single girl maturbation scenes and a few girl-girl scenes and it sells like a mother fucker.
You can make any niche make damn good money if you know how to market it right and get the correct type of traffic to it. Don't fall into the trap that extreme automatically means more money. It very well might but it also carries more risk.