The basic premise is that one needs 18kohms across the A & B wires, on the telephony side of a filter. It needs to be easily removable; if a fault is reported, I don't think a BTOR technician would appreciate chasing 18k of 'low insulation' that turned out to be in your house.
I could fabricate this thing from an RJ11 adaptor, or a rat's tail ADSL filter - it all rather depends on your set up.
I gather from previous posts that you have a central filter on the master socket. Do you have any extension sockets, and if so, is there a 'spare' one with no phone.
If there is a spare extension socket, the adaptor option would be the neatest solution. Alternatively, it would mean a socket doubler (I have a spare one) feeding the phone and modified filter.
The first option would go as 'large letter' - first class 96p or second 75p.
Second one would be 'small parcel' at 5 first or 5 second.