what is realistic to the UK is tankers, high volume pumps and hose layers which can come as their own unit or as a pod on the prime mover system.
Putting water limits on our engines is unrealistic as in most circumstances we have a unlimited supply via hydrants and water courses.
In the urban environment almost every street has a hydrant even in the rural communities they have hydrants avaliable.
Also out in the rural areas if there is a major fire, we can draw water from almost any water course and either ferry it in tankers and spare engines or they would set up a pump relay or get a HVP and hoses.
A big example of real life working of getting water to a scene would be the Lockerbie disaster, that was a urban environment but due to how the aircraft crashed it destroyed the water mains that supplied the town which made water access a problem.
They did have hydrant supply but at a lower than normal pressure and it wasn't enough so had to find a local water course to use, which in that area there isn't a major river or loch to use, there was however a small stream they had to dam and use to fill tankers up to ferry water in.
Yes water supply was limited at first but was soon only limited by getting it from the source to the scene.
There have been several other big incidents here in Scotland where they have used both Hydrant supply and water course supply.
Notably Glasgow School of Art, they were drawing water from every hydrant near by and also drawing from the Clyde using the HVP so water supply was more or less unlimited.
Very rarely you would get a call where water supply was limited and when you do there are the tankers to get a good start until a supply can be made or set up ferrying. Yes there will be calls like wildfires where you truly are limited but depending on risk these are managed to contain the spread and left to burn out.