Which boiler system for a 5bed, 3 bath house?

[Propertyquandry]

Finally getting rid of our old inefficient boiler. I want the new one to be powerful and be able to cope with DH, myself and 2 teenage boys showering in the morning, often 2 or 3 of us at once. Ds3 and DD have an evening bath and ds2 often another shower at this time after rugby or similar. I don't want the bath to take a half hour to run either.
I gather a combi is out but what do I actually need. And what's a megaflow. TIA

Marking place... We have just moved from 5 beds, 2 bath + 2 WCs and a perfectly capable combi (Worcester) to 5 beds, 3 baths + WC and an arcane, impossible to understand heating system. We know we have an ancient Camray oil-fired boiler and a Megaflo cylinder, but that's as far as it goes. Trying to work out what panel actually controls which radiators/underfloor is a nightmare, but I suspect I'll be starting a new thread on that soon!

We're going to be doing building work - can a Megaflo be moved downstairs to the boiler room or is there a pressure reason why it needs to be upstairs? It would seem from this thread that we need to keep both...

We had the pressurised system installed for our 4 bed, 3 bath house and it's fab, it's definitely a separate thing to the boiler as we had the boiler replaced the year after - any plumber who comes to our house loves he joint system we have and we never run out of hot water 😊

A pressurised cylinder can be anywhere, but, please, not in the loft. It does not need to be upstairs. It is well insulated so does not leak much heat. It has to be accessible for regular inspection and occasional servicing.

When full it will weigh in the region of 300kg

There is a slight advantage in economy if it is close to the hot tap you use most frequently. This is often the kitchen.

We have a giant mega flow in flat and it's on the top floor, wouldn't have anywhere else for it to go and flat is one level. We don't have a boiler at all in flat