Fred,
The SSII horn relay is mounted on the fender inside the engine compartment, for which British Wiring's part #819 would be perfect. However, it looks like the SS horn relay is mounted on a circuit board, which means you don't need the mounting ears on the can, and you do need the location of the pins to match (they move around between some of the different relays: the "6RA" family includes at least the 33223, 33269A, 33270A, and 33271A).
Some other options:
1) Overhaul the existing relay. (Do you know what's wrong with it? Pitted and/or misaligned contacts are easy to fix, loose pins a bit harder but still doable, while a burned-out winding might put it beyond economical repair.)
2) Get a salvage part from Paul at CMC, or from some breaker yard in the States.
3) Order the correct 6RA relay (RR part #UD11469) new from Flying Spares, Montague, etc. (This option will be eye-wateringly expensive; expect to pay north of $80.)
Oh, one more: call up British Wiring and find out if the C1/C2 and W1/W2 pins on their part #819 are in the same place as your horn relay, and if they are you could get one and then swap cans (take the guts of the 819 out and put it in your can).
Jeff.