They are, but that’s largely symbolic. The UK only has a few hundred Challengers to begin with.
Supposedly Germany wants the US to go first with Abrams before authorizing the Leopard for release (export) from various other countries to Ukraine. It seems that it’s largely agreed that the Abrams is a poor fit for Ukraine. Running on gasoline is a huge reason why (what were we thinking?!)... But if giving them 50 Abrams (or whatever) breaks the LEOPARD 2 logjam, it’s worth it. Ukraine can position the Abrams in the western part of the country where jet fuel supply lines will be manageable (plus they won’t need to move around much anyway) as a defensive/reactive force to deter Russia from attacking towards Kiev again from the north.
The vehicles that appear to be in largest supply, and would be the best fit for the frontline war, are the US Bradley and the Leopard 2. We have Bradley’s coming out our ears. Western Europe has a lot of Leopards- thousands. Both run on diesel.
Bradleys are gonna be plenty potent against the T62’s, T72’s, and T90’s Russia is fielding. The TOW missile is a tank killer. It’s mounted up high along with the excellent Bradley suite of sensors/sights, so a Bradley can be largely behind cover, yet be fully on offense against tanks. Their small bore high velocity cannon, paired with excellent sights, will tear up the Russian infantry fighting vehicles- the various BMP’s. They’re going to make a huge difference. I don’t know much about the Leopards, other than that they thoroughly outclass everything Russia has available in quantity. They’ll be destroying Russian armor that never even knew the Leopard was there.
Get a few hundred Leopards, a few hundred Bradley’s, delivered and in service and Ukraine wins this war going away. Unless Putin goes nuclear, which he might.