Thought I'd answered that in post 9?
Specifically, if you were using 28mm on the mains water supply, all that would do is improve the working pressure (i.e. pressure that there is in the pipe when there is an x litres per minute flow), and it may not improve it very much or even in any way that you would actually notice without laboratory test equipment. You might use 28mm if it were a very large house or number of flats, or something like a hotel where lots of people might be drawing water at once due to there being 5 bathrooms or whatever. Very unlikely in a 1-2 bathroom town house.
As far as radiators are concerned, the pipes should be sized correctly. Having oversized pipework would just increase the amount of water the boiler would need to heat before any heat actually reached the radiator. If your radiators are all taking an excessive time to heat up, the issue is more likely to be a pump issude or undersized boiler or, if the problem is just some radiators, the system probably needs what is often called 'radiator balancing' which is "just" a number of adjustments to valve settings.
Finally, as you refer to showers, I'm assuming you're talking about hot water. As you have a combi boiler, the pressure will be affected by the pipework diameter, but in much the same way that cold water is affected. What is the specific problem you are experiencing that you are trying to solve? My bath hot tap is run in 22mm, but as my hot water system is a vented cylinder, I have less than a third of the minimum mains pressure you are likely to have and I still get 20 litres per minute. No combi boiler will give that sort of flow as you are limited to how much water the boiler can heat in real time (unlike with a cylinder where the heat can be accumulated for later use) and, if it did, it is unlikely to be very hot water anyway.
