Hello,
I don't think switching this mortgage would differ greatly from switching an ordinary home loan mortgage i.e. cost of breaking a fixed rate, ordinary LTV and repayment calculations carried out by the new lender etc etc.
In terms of banks paying switching costs it really is all about the dynamics of the market, size of the loan etc. If you have a buy to let loan with a large principal outstanding then I'm sure the bank will compete strongly for the business i.e. they may pay remortgaging legal costs even if they don't have a published policy of doing so. Buy-to-ley mortgages aren't as widely advertised as loans for owner-occupied houses so you will be unlikely to see an ad for a buy-to-let mortgage including payment of remortgaging legal costs but that doesn't mean they won't do it if there's money to be made. I would suggest you find out the participant banks in your marketplace and start describing your scenario to them... that's the only sure way of finding out!
Regards,
INBSMember