The Mk1 Focus RS is already a pretty rare car with only circa 2100 UK RHD models from the factory. To put that into perspective iirc there were over 8500 S1 Escort RS Turbo's built, so there are less than quarter to begin with and you can see the values of a clean S1 these days. After you take account of all the ones written off and there are afiar few Cat C, D's how many are actually left. Maybe 1500 or less?

It will be a modern classis in 2/3 years time and you will be able to get them insured under a classic car policy for something crazy like £100.

Cant see them ever being as special as say a 3 door Cossie, but at least it wont spend half its life getting repaired as its a fairly modern and reliable car. A shortage of RS specific parts is where you might struggle, but maybe the RSOC will pull their finger out and actually do something in this regard.