Nothing initially. A fairly sensible gamble unless you can really foresee the uk going it alone against a near peer adversary within the 10 years it would take to buy an off the shelf replacement. Poland is purchasing/ license building 1000 Korean K2s.
Why hold on to 150 Challenger 2s to convert them to Challenger 3, to maybe deter Russian armour in 5 years time when those same Challenger 2s can wrap up said Russian armour this spring?
It looks nice having a park full of tanks, but if you aren’t prepared to use them when needed incase you loose them, when will you ever use them?