You seem to be over-thinking things. The reason that there is not a UK equivalent of a US 2257 is because 2257 is not a UK law. Its only for the US.
The flaw in your plan isn't going to be to convince girls to perform on cam - Its going to be to convince them to give YOU all their personal IDs and details. I mean, FUCK THAT! they would be crazy to give you info like that which you don't need and isn't a legal requirement in the UK
As for model release I don't think its strictly necessary, if the girl is taking the pics and passing them on to you. The only thing a release allows you to do, is to use the pics for whatever you want, regardless of what the model may or may not like you to do with them, as effectively they sign away and and all rights to the pics in return for payment.
I doubt that applies in your case though, as if you are only gonna use them to promote the girls and the girls know that and have given you the pics for that exact purpose, whats the worst that can happen? The girl changes her mind and doesnt want you to promote her or use her pics, then thats fair - You just take them down... Sure you have wasted your time, but welcome to being a middleman in the buisness.
If you wanna be 'sneaky' use this form, which is a model release I have used in the UK myself. It includes the relevant 2257 sections as I sold my work to the states so I covered myself. Download it here -
www.leocaprice.com/docs/adult-model-release.doc - and hardcode your info into it and take out the payment parts. Simply have the girls complete it for you as a formality, and that would cover you in the event that a girl wanted you to stop using her pics and you decided not too...
But we are heading into a murky area that doesn't need to be explored. Just be up-front and honest and you will still get problems but the problems will be easy to overcome. Like I say, don't over-think and overcomplicate things. The reason you cant find certain info is cause it doesn't exist as it has no need to exist.
Anyway hope this helps at least a bit
