replacing hot water pipes to new bath (15mm v 22mm)

[dandt]

My old bath is served by a 22mm hot water pipe (historically fed from a gravity system I think). Anyway it is now served by a combi in the next room (downstairs kitchen) and hot water from combi is 15mm which then connects to the 22mm pipe in bathroom. I am thinking of replacing all of the water pipes in bathroom for various reasons to do with layout. I am guessing that I can use 15mm for hot throughout as the present 22mm does not increase flow because of the 15mm beforehand. Am I right to use 15mm which would be easier for me?
Also can I use flexible plastic for hot water?
I will also be fitting a mixer shower (and I know that there are potential problems ahead eg I may have to reduce my mains pressure but I'll be optimistic for now)
Finally I am reasonably competent and have done two kitchens in the past. I usually get by with copper and compression joints but I want to give plastic a try this time.
Thanks

 

[Whitespirit66]

Gravity relied on flow of water so larger pipes needed. Combis are fine on 15mm, as the water is under pressure.

Yes, plastic pipe is OK with hot, but make sure it's barrier pipe if you use it on pipes for central heating.