Looking on various sites it appears sub-prime mortgages was not giving correct advice or cheapest products.
Certainly a sub-prime mortgage wont be the cheapest product. By definition, sub-prime mortgages are for borrowers who are a higher than average risk or who have a less than perfect credit record and so interest rates will be higher than they would be for low risk customers. While with PPI, it is almost automatic that, when you ask, you get compensation or a refund, with another product, you would need to prove that the advisor or the provider was negligent.
It sounds like your dispute is with your advisor. If you feel unable to persue a claim on your own and if you feel that there is a genuine claim, I would suggest that you gather as much information as you can and make a fixed time appointment with a local solicitor who specialises in this area. You might pay £60 for ½ hour (agree this in advance) but the advice is likely to be a lot better than you would get from a company that makes unsolicited phone calls.
My gut feeling is that the person whom you spoke to was trying to con you and there isn't £10k just waiting for you to collect it.