If we're serious now rather than virtue signalling, why aren't we training crews on and supplying our remaining MBTs, we've no reasonable use for them (force projection by threat?).Buggered if I know.
National pride.
The fact that Britain, who once produced and globally exported some of the best tanks going no longer has the ability to build tanks is something our government believe to be of high embarrassment for the nation, or would be if the public knew. The government are currently telling people that we have a new tank called Challenger 3 coming when in reality what we have coming is a German upgrade package for our existing Challenger 2 tanks, they've gone so far as to create a British/German shell company to give the contracts to in order to try and hide the fact that they're just getting the guys who build Leopards to upgrade our existing tanks to be more Leopard like.
If we gave away all our tanks and had no tanks the country would look both silly and weak, both of which would would make the government look bad, and you can see in the above paragraph the lengths they're currently going to to avoid looking bad (and to avoid actually investing in British industry but hey ho).
They left Afghanistan and had suffered far fewer casualties and much less economic damage.
It wasn't the casualties that caused the problem for them in Afghanistan, it was the time, nine years of achieving nothing but dead Soviets.
There's some very simple maths here. Building a replacement soldier takes ~18 years, building a tank ... a lot less.
To add to this, a lot of people try to think of the Russian army and instead imagine the Soviet army, but the two aren't really comparable, the Soviet army was better trained, equipped, commanded and also importantly when charging into bullets: more numerous.
People tend to mis-use the USSR and Russia as interchangeable terms but it's often forgotten that while Russia is the accepted successor state they only mad up half of it's population.
For every three Russians in the USSR there was one Ukrainian. Russia cannot maintain their current infantry loss rates, they may be able to absorb/replenish their equipment losses for years but at this rate they will run out of fighting age men before Ukraine does (theoretically of course, no country would actually keep fighting to that point, even Russia).