Merchant account for 3rd party processing apps?

I'm currently trying to get a merchant account for my business that's essentially a 3rd party processing app for a large niche market here in the UK, however when I explain the nature of the business, the banks simply turn us down flat.

Even Paypal turned us down for our WPP application, though they were more vague with their reasoning.

Obviously things are stalled until we can get this issue sorted :)