Replace thermostat with rasberry pi and Solid state relay
connect Red (L - Red) to SSR contact 1 (ac in) and CALL (yellow) to the SSR contact 2 (Load)?


Yes, that seems correct. You should make safe the neutral wire and prevent it from any possibility of coming into contact with any other wiring, metal parts or the outside.


  It's in the UK, so voltage is 220V


The EU standard is 230 V ± 10% but in the UK you'll typically measure 240 V which is within this range and is what substations for UK domestic supplies were previously standardised on. Other EU countries had previously standardised on 220 V, not the UK.


  I want to be extremely careful


That means going to the consumer-unit (fuse-box) and switching off the circuit marked "boiler" or "heating", then checking your thermostat connections with a safe voltage tester (preferably non-contact) before rewiring them. keep the Pi well away from 240 V wires, ideally in a separate, well-insulated box.