Your a little confused with the forms / terminology here.
Your paid by the agency, they deduct from your wages tax (termed PAYE) and National insurance. Part of your rights is that you are entitled to holiday which you accrue (or earn) as you work for them.
The form they use will be internal, P11D is a tax form for different forms of income during your employment but I doubt its applicable to you. They will issue with a P45 when you leave but you want them to sort out holiday your are owed in your final pay run for them before they issue the P45.
Basically, ask them what forms you need to complete to find out how much holiday you have accrue whilst working for the agency, then confirm that this amount will be paid to you in your final pay run. If they say you can't have it as cash then tell them you want that many days holiday next week or whatever before they end your employment. The completing forms thing will be a red herring really just to make it less likely you apply imho, as the payroll software they use will know exactly how much holiday you are owed.
I have had agencies here that have never mentioned holiday entitlements at all, its quite naughty as they are conning the agency workers out of money they are entitled to.