Mortage interest and other expenses

A client rents two rooms in his three bedroom home.  Rental income is circa £20K and the client pays circa £15K in mortgage interest. Can he claim relief for 2/3rds of the interest and lighting and heating, or is he limited to £7.5 Rent a room relief?  If he can claim the actual proportion does that disqualify him from PPR Relief on that portion of gain on sale of the property? I'm leaning towards a loss of PPR Relief if he claims the actual proportion, but I've not come across this problem before.