Tanks don't equate military equipment by and large. The amount of Western tanks Ukraine has been promised indeed has been small, but that doesn't equate with other military equipment. They received far more artillery and anti-tank missiles for instance. (By one account Ukraine has received at least 264 Western tube artillery pieces, for instance, and "hundreds of thousands" of shells for those. And at least 5,000 Javelins.) Or even many more short range anti-air, but not so much longer range anti-air, at least not of Western make, like Patriots.
TBH, I personally find somewhat questionable linking [Challenger 2] tanks to somehow stopping the Russian Kh-22 (cruise missile) bombardments. Even if Ukraine takes back 100% of the territory they claim [using their new tanks], it's not too clear to me that would stop Russian cruise missile attacks. As one retired US general said, giving ATACMS to Ukraine to hit back much further into Russia would be more of a levelling plan, in that regard.
Yeah, there's the reluctance in some Western political circles of what equipment to give to Ukraine without aggravating Russia too much, and possibly causing them to hit the nuclear button and what not. This is reflected in some NATO think papers on the topic.
As for tanks in particular, as was the case with artillery before, it appears that planners preferred to first exhaust the stocks of former Soviet equipment, before sending Western made ones. According to France24 "Ukraine's European allies have sent Kyiv more than 300 modernised Soviet tanks since Russia invaded more than 10 months ago." The most recent, sizeable batch so ordered appears to have been from the Czech Republic, back in November (some 90 T-72s).
And it takes none other than Ukrainian journalists to point to the scale and relevance of Russian armored losses in Ukraine:
According to the 2022 Military Balance report published by the International Institute of Strategic Studies, Russia started the war with 3,417 functional tanks, of which 2,357 were built after 2000 and more or less modern.
Military journalist Yury Butusov, in a Nov. 21 morning Facebook analysis, estimated that of those tanks lost to the Russian military, at least 516 were expensive and relatively up-to-date models like T-72B3, T-72B3M, T-80BVM, T-90A, and T-90M, equipped with high tech thermal sights and, because of western sanctions, almost impossible to replace.
So according to that analysis Russia lost about 20% of their modern tanks in Ukraine. And this resulted in a change of tactics:
Volodymyr Lysovsky, a fighter and anti-tank gunner in a Kyiv regional territorial defense battalion, told Kyiv Post in a November interview the Russian military entered the war using World War II tactics of massed tank columns that, in theory, would deploy overwhelming firepower and impenetrable armor to overrun defenders.
That aggressive, offensive style of war, successful in past Russian offensives against poorly armed infantry in Chechnya, Syria and Georgia, over and over drove the Kremlin’s tanks into ambushes by Ukrainian infantry heavily armed with anti-tank missiles fully capable of blowing up any armored vehicle in the Russian army inventory, Lysovsky said. [...] “The Russians for a long time threw away tanks like they were washrags, but they don’t do that anymore,” Lysovsky said. “Now I guess they are running out.”
[...] The Russian Federation (RF) intended to produce 240 modern tanks in 2022 to add to its fleet but, because of Western sanctions on Russia, “that plan failed completely,” Butusov said.
And that Russia is apparently looking to refurbish a further 800 T-62 to more modern standards is also discussed therein (based on Russian sources). However, those numbers are far from the tens of thousands bandied in the OP's post.
That Russian attacks [for] now are dominated by (Wagner) infantry and artillery has been noticed elsewhere. Now one shouldn't read too much into this as carefully marshalling armored reserves for surprise attacks is as old as WW2, and Ukraine's leaders have talked about potential for more serious Russian offensive in 2023.
However, attacking tanks in a defensive position, especially over open country, is rather more difficult than if they come to you, so it is understandable that Ukraine seeks to improve that kind capability now.
As for relevance of UK's MBT announcement... sadly [for Ukraine] it wasn't too consequential as far as Germany goes as they seem to also condition their own provision of tanks on what the US does (and that's relevant because far more Leopard 2's have been produced and sold to various European countries, compared to Challenger 2's), although
there have been a number of somewhat contradictory remarks from Germany on that, with the conditionality on US shipments being denied in somewhat vague language more recently.
By the way, those 50 or so Bradleys were soon accompanied by pledges of a similar numbers of German Marders (40 in Q1) and French AMX-10RC. So one might guess Germany is ideally waiting/hoping for a similar kind of agreement on main battle tanks. The US DoD however is keen to point out that while Bradleys (and most European main battle tanks) run on diesel, their Abrams tanks [normally] run on JP-8 aviation fuel, which would be logistically more problematic for Ukraine.
There's no denying however that once a couple of the big NATO players send some sort of equipment to Ukraine, others in the coalition seem to follow. Case in point, Sweden then announced sending some 50 IFVs of their own (as well as 12 Archer artillery systems--one of the most modern in service anywhere, in terms of automation at least). The Western IVF insofar pledged (around 150 now) are definitely superior to the last large batch of Soviet-era IFVs I remember Ukraine got, meaning those former Czech & East German BMP-1s that were mothballed in various places in Europe (Czechia, Sweden, Greece, Slovakia). And in terms of gunnery, the fast moving AMX10-RC [which doesn't carry infantry] is also superior. One should remember that Russia also has some units (paratrooper) that emphasize mobility and don't have that much in the way of heavier tanks, but rely on BMDs. And a bunch of these were deployed to Ukraine.