I'm not a fan of changing belts due to "age". I've changed plenty of belts at 3, 4 and even 5 years old and there's never any physical difference. Your 20k belt will look brand new.
The "3 years" thing might be sensible for a belt that lives in the desert or in some hot and dusty Mediterranean place... but the UK has a cool and moist climate which is the best possible conditions for rubber parts to last forever. I reckon your belt would do 5 years easy.... but then it's not my engine.
You could take the cover off and have a peek at the rubber teeth. If they have no wear on them and the toothed surface looks supple and shiny.. .and you have no cracks or scores on it, then that belt is fine.
Water pump is a bit of a punt too. My 145 pump did 72,000 miles easily and it was almost a pity to throw it away... but I didn't want to leave it in there for 108,000 (the next service). So, I'd personally change my pump with every other service.
But 72k is a piece of cake for it, so you could arguably leave yours until the next service.. at your mileage (20k in 3 years) your pump will have done 76k. No worries.
If you leave your cambelt for 36,000 miles rather than 3 years.. then your pump will have done 92,000.. probably time for it to hang up its boots for sure.
Variator should be good for 108,000 easy. Mine is a tiny bit rattly on 120k so I may
change it at the 132k cambelt change... but yours has plenty of life in it. If you decide to remove it, I'll have it off you..