Some agencies are 'contract' agencies, which means they will offer you a contract, pay for your portfolio, etc. However, in most instances, they would hold exclusivity to represent you and receive a percentage of your earnings.
The UK has a BIG problem with scam artists chatting up people, saying they have potential etc., then asking them to pay registration fees, portfolio fees, etc. Under no circumstances should you do this. If you don't have a portfolio and you seriously believe you have potential and it's a career you wish to pursue, then I'd suggest having your portfolio made 'away' from any agency. That way you can post your headshots, etc directly to any agency for consideration.
And by NO means should you go to one of those shopping centre 'Glamour Shot' type places to have a portfolio made. That is not what you want. You'll need photos that show your diversity, not putting you into a set pattern. I have too many friends in the industry and I've watched them either chuckle or shake their heads as they tossed those 'glamour' style photos into the dustbin. I'm sure that is not what you wish to have happen.
I do wish you the best of luck
Fr Bill