We are going to be replacing our existing (old) conventional boiler as part of extension works we are carrying out.
We are aware of the pros and cons of the main boiler types (essentially combi v's conventional / system).
And in our 2 previous houses, we have put in combi boilers both times – this is because the pros of being able to get hot water on demand and the boiler taking up less space (both very important to us) have far outweighed all else. However, we have previously had 1 bathroom / 3 bedrooms / 1 reception / kitchen, and a baby and a toddler - so the limited hot water output (ie running more than one hot water tap at the same time) has never been a problem.
Our new (post extension) house will have 2 bathrooms / downstairs loo / 4 bedrooms / 3 receptions / kitchen-diner, and the children will be growing up here…so our question is essentially whether there are any combi type boilers out there could cope with this…???
I have seen some that suggest that they might be suitable, for example: the Worcester Bosch Greenstar 42CDi Classic that says ‘The higher outputs of Greenstar CDi boilers can be suitable for larger properties with more than one bathroom’; and the Alpha FlowSmart that states “allows a sustained supply of domestic hot water at flow rates of around 18 litres per minute.” And presumably others.
But does anyone have any experience of what this really means in terms of say, the central heating being on and 2 people showering at the same time?
I think the acid test would be either (a) central heating on, one person showering, another person then starts doing some washing up – does the person in the shower lose their hot water? Or (b) even more simply, with the central heating on, if 2 showers are running at the same time, will they both run hot?
[I don’t think the following will be likely (and if it does happen, then someone would just need to wait!): central heating on, 2 showers happening at the same time as well as another hot water tap running. And we are talking about ‘normal’ shower heads by the way, not those oversize ones. And regarding the other potential draws of hot water: the dishwasher always goes on overnight, and our washing machine is cold feed. Also, although not confirmed yet, I think there will be approximately 16 radiators in the house. And finally, our mains water pressure seems decent.].
Any help would be appreciated.. thanks..!

The output (flow rate) from any instantaneous water heater (eg combi boiler or electric shower etc) is limited by it's power rating (kW). So statements like "our boilers produce a high flow rate" are nonsense without knowing the power rating of the boiler. For example a 20kW combi boiler can't magically deliver better flow rates than say two 10kW electric showers as the total power available to heat the water is about the same.

A combi boiler is not the only way to get mains pressure hot water. There are various types of hot water cylinder systems that will give you mains pressure water at a flow rate that's limited by what the mains can deliver rather than the power of the boiler.  

It's a shame but many people with old gravity fed systems are upgraded to a combi by their plumber without being told about or offered a mains pressure stored hot water system (of which an unvented cylinder is one type). They are worth investigating if you need very good flow rates.