Is the gravity hot water slowly fading out in the UK?
Ever so gradually, but some properties will have it forever. If you've got a straightforward oldfashioned tap and a floor or more of head, then the flow is ok, so most people are happy. Fancy taps are a pain, even a combined single lever one designed for "low pressure" won't have great flow from a tank on the floor of the loft above, whether it's fed by 15 or 22. It's the tap that matters. "Designed for 0.1m head" means little if they don't quote a flow rate.
That 3-4m lengh of pipe is not likely to make a difference you can see, though you
may be able to measure it, would be my guess, though I don't know the nubers you'd have.
I thought I'd get my calculator out as you're an old timer here masona
...
assuming a lowish flow rate of 5 litres per minute, you'd have a head loss
per metre of ~ 2.5cm with 15mm, and 0.4cm with 22mm.
3m, 3 elbows, makes maybe 10cm head difference
Not enough to show much, if your pipe head's say 2.5 metres.
A better tap would mean more flow so the pipe makes more difference, but with better flow, you'd care less!
By the way some tap flexible connectors are quite restrictive, but again not usually too significant compared with the tap itself.
22mm on a combi means it takes longer for the hot to arrive, and you waste more heat when you turn it off, though often the house gets it.