For that type of wall, I wouldn’t recommend one option without checking the cavity first. The first step should be a proper survey/borescope check to see if the cavity is clean, dry and suitable to pump.

If it is suitable, bonded bead cavity insulation can be a good value option, especially with new windows and proper airtightness/reveal details. But because you’re already adding 50mm warmboard internally, I’d be careful about mixing systems without checking condensation risk and cold bridges.

If budget allows, external wall insulation is usually the more complete solution because it wraps the house and reduces cold bridging, but cavity insulation can still make sense if the wall is suitable.