A few tips:
Unless you have a cold water storage tank, the amount of water coming out of the taps (all the taps added together) at any one time cannot exceed the amount of water coming into the house through the supply pipe. This is the flow.
In older houses the supply from the main under the pavement is usually a half-inch or even 3/8" pipe in lead or steel, or a 15mm pipe in copper. You can get an idea of how much flow it delivers by filling a bucket at the cold tap at your kitchen sink, and at the garden tap if you have one, timing it, and calculating how many litres per minute you get. It will probably not be enough.
Lead pipes last about 100 years, sometimes 2000, steel pipes last about 40 years. It is difficult to predict reliably when they will start to leak. If you are upgrading your plumbing it is usually a good idea to replace the supply pipe in 25mm blue plastic, or 32mm which costs little extra, all the way to the water meter under the pavement. Fit full-bore stop valves or you will be constricting the flow (some plumbers little ones as they are cheaper). If you are contemplating replacing the pipe, and it is lead, get the water co to test the drinking water for lead content well before you start. There may be a lead replacement subsidy or they may connect the new pipe at a reduced price. If you don't do it now, then one day, you will wish you had.
Try the flow test in your old house. Then fill a bucket at the hot tap and the cold tap on the bath. It will probably be more flow. Consider that a bath holds about 100 litres of water, and calculate how long it will take to fill.
Flow is not the same as pressure.
It is useful to know that energy from electricity costs three or four times as much as energy from gas.
If you have a hw cylinder with one or two immersion heaters in it, you can continue to have hot water when your boiler breaks down.
Combi boilers break down more often.
Solar heating of hw is not an economic proposition unless you have no gas.