Follow Barmaid's advice - she's a barrister & knows what she's talking about!
You may well need sworn statements (either as affidavits or statutory declarations) from the witnesses confirming what was said.
The loan you signed for went to an account you were an authorised user of. That means you could sign cheques on it, I suppose. That, & the fact that you were asked to sign the loan seem to indicate you were not an employee but a business partner (your first description). Was there any formal partnership documentation? Was there a trading name? Was the loan given to the partnership (i.e. made out to the trading name)? If not, was it given to both of you by name? Were the loans where your signature was forged made out in the same way? Were those loans paid into the same bank account as the first one? If so, did you realise the money was there &, if you did, what did you do about it?
These are some of the questions that will come up in trying to sort this out for you, so be prepared with answers before you see a solicitor.