Closed panel lenders?

Should mortgage companies be able set the requirement to use a closed panel of solicitors as one of the criteria for obtaining a loan from them?

I have a cousin who has his own legal practice in my area and there are a significant number of partners, enough there for any lender to allow them to be used.

Some BTL lenders give you a list of about 10 solicitors firms that they insist you must use to have a loan with the company.

So instead of instructing my cousin, which would cost me about £800 per purchase, I am given a list of solicitors I need to use to complete the purchase. These solicitors are given a fixed fee they must charge, by the lender. The cost is then hiked to about £2700 per transaction.

Feel this is the making of a cartel, but unsure of the contract law behind thee transactions.

Has anyone else run across this as an issue or have any advice?

Many thanks

Wayne