New Old Stock bellows sleeve thermostats quite often on Ebay UK, for the XKs,
NOS correct bellows sleeve thermostats are on Ebay quite often: a bit hard to find, have to try several searches…,try “bellows sleeve thermostat”, try Smiths X85025 , or thermostat TF1, or TF2,
also must look up on Ebay UK. Usually $40-50USD. You want 68C, or 72C to 80C…but prob not the 86C tho it could be used.
The same thermostat is for all XK120, 140,150 and early S1. Must be have sleeve…not bare bellows, must use 4lb radiator cap. Even a 7lb may harm the stat. 3lb only provides a few deress of boil, just increase anti-freeze mix by 10% or so…from 50-50 to 60-40 or 70-30 max…much more boil temp…still the 4lb cap.