If you are IFR off airways in the UK it is probably better than the FPL is addressed as if it it is VFR. Addressing it to the two IFPS addresses is correct but that will not faciltate crossing clearance of any Class D you may find along the route. Neither is sending it manually to any en-route airfields. Few, if any, will take any action upon receipt, but would rather wait for the 'free call' when you call. Sorry, but it just the way the system has been 'brung up'.