Well here in the states any 00-07 focus seats are interchangeable and most people agreeing that the seats from the SVT are more comfortable than the stock seats, especially the Recaros.
So, see what you can find in a junk yard/locally. I'd probably guess the seats from an ST170 would be a nice step up. I'm not familiar with all of the UK mk1 trim levels to know if any other models had better seats. Seats can be pricey though, just depends on where you find them. You can always look at after market seats and fit them to your car as well.