The UK has a rather convenient batch of Typhoon (Tranche 1) fighters that are due to be retired in 2025 - I think it's about 40 airframes in total, but a good number (half?) have been cannibalised to keep the others in the air.
Tranche 1 was a bit of a cul-de-sac - after they were produced, primarily for the Air-to-air role with an 'austere' air to ground capability, the design changed significantly and it's simply not possible to upgrade the Tranche 1 into T2/3/4 etc... Obviously it's possible, it would just require a rebuilding of the airframe to the point where buying a new T3 would be cheaper...
20 Typhoons would help Ukraine, no doubt about it, but they aren't going to end the war. Their value - rather like the Challenger tanks - is to 'break the mold' of hitherto western support.
The really big issue that the west is going to have to face is that without 'safe havens', Ukraine is going to struggle to operate western fighters to their fullest potential - for me, if we want Ukraine to win, and to do so as quickly as possible, then Ukraine needs to be able to base it's aircraft in NATO member states, free from Ru interference. That's a very big step, though it should be remembered that both Russia and China have hosted other states belligerent aircraft where the US couldn't get to them, and it didn't start WW3...