Newbie help - shower feed size etc

Hi all, i'm moving the bathroom from downstairs to upstairs and need a few professional pointers

I've read that bath 'fillers' and showers should have a 22mm feed, how important is this?

I'm not too bothered about the bath but the shower i want to give it the best chance.
The hot water tank/cylinder is on the same lever as the new bathroom and the cold tank is in the loft, all be it on the floor so i may need to lift that up to increase pressure?
My questions are.
The shower mixer i have is gravity fed and has 15mm connections so my questions are.

1.Should i just run 15mm pipe to the shower or run 22mm and then reduce it almost at the actual shower, the reason i was thinking this was because would it retain more flowrate and pressure?

2. If to go for just 15mm feed would i be able to take the feed for shower hot from the general hot feed in the bathroom ie. same pipe feeding sink, bath and shower, i would it be better for the shower to have its own feed (both hot and cold)

I'm going to instal it so that i can easily add a pump if needed but i'd prefer not to due to cost.

Any info/advice/pointers would be great and i'll thank anyone now in advance, i will just mention thought that i currently have no internet connection so i wont be on here again until midweek probably so please bare with

Many thanks all

Gav

Also, sorry if this is a repeated thread, i've read alot of threads in the shower/wetroom but none seem to fully answer my questions,