Hmm!
Not really sure what the question is, but the likes of combis and unvented require a fairly high mains pressure.
You could of course use a water accumulator/potable storage tank and pumps but they are dear.
But then if you use a low pressure stored water system, you've also got to get the water up to the water storage tank in the loft or high point.
One of the reasons they used indirect low pressure hot water systems was to avoid limescale build up in hard water areas. In the direct system the water in the hot water heating loop between the boiler and cylinder was always getting replaced by new water so increasing limescale build up.
The indirect system meant this did not happen so often as the same water kept circulating and so you could say the water in the closed loop became less limescale prone. Also you could dose it with inhibitor knowing it should not mix with the domestic water as the direct water does.
I would say though, that if you have got low mains pressure below say 1 bar at 9 litres a minute flow, you should contact your water supplier and ask them if they can do anything.
But if for instance your water main is old and runs on private property for any distance, the chances are it may be damaged, clogged or any of a variety of things. If that is to expensive to get repaired, then without paying out for boosters it would seem stored water is the only choice. That is of course if you can get it up high enough. If you can't, you may have to go for individual over sink/ bath heaters. But once again you may need some pressure for these.
Basically pressure is the waters energy. So if you connect something to a water main that has loads of resistance in it, you need pressure to force the water around it so it comes out. That is why if you connect unvented equipment on low pressure mains you can get loads of problems.
The likes of combis can have a maze of pipes inside them to heat the dhw, don't forget it comes in cold off the main and has to go out very hot virtually instantaneously. So to do that it has to be exposed to as much hot water heating surface as possible in the heat exchanger.
That usually means loads of pipes or plates. They are also usually small pipes/plates to allow the gas burner flame to get to work and put its heat into the water quickly.
A bit like its probably easier to heat a litre of water than it is to heat 2 litres or dry up a pool of water 2mm deep than one 6mm.
Combis work on the same idea. Keep the hot water heating area big and the amount of water being heated small. That is why, the bigger the combi the more hot water flow. Simply put its got more heat output so you can increase the pipe/plate sizes inside making it easier for the water to flow.
Anyway I am wondering off and probably not even begun to answer your question, I better stop now.
