Britain needs tanks, and an aircraft carrier or two, so it can strut about the world world stage pretending it's still one of the big boys. It has a bit of an arms industry as well, so flogging the hardware is a nice little earner which needs a shop window.
Yer man does raise an interesting point about the Challenger in Ukraine. From being the hailed as the latest game changer that's going to knock the Ruskies overnight it is now never mentioned. Is there a fleet of them parked up somewhere, rusting gently away as the UK has asked them not to be actually used as intended because they are not up to the job? Very possibly, and the fact that that military instructors are not going out any time soon suggests that Ukraine won't have crews trained for them anyway. Perhaps it's not the tank's fault they can't be used after all, it maybe the fact that the guys inside need to know what they are doing, and that can't be instilled in them over a weekend. It's all a complete and quite avoidable balls up whichever way you look at it.