I got an original old stock key to match my locks by googling Wilmot Breeden FP654
https://planefacts.co.uk/keys/
Now that you have mentioned it I just googled FS and FP keys for the UK and got a whole load of possibilities - plenty out there.
Just got a surprise, whilst I was looking for that key that CIA obtained for me I looked at the keys I found under the dash, one was an FP and the other an FS and both work the ignition? and there is a slight difference between the two.
Bob