@CBW I'm moving to combi so the issue was not about main fed cold/gravity fed hot. I'm asking normally when converting from traditional boiler to combi, do people move their pipeworks from the loft to under the floor? This has big implication for my renovation plan.
What you class as traditional is still a current model. Combi is all in one boiler as complexity is what will make sure you have no hot water or heating when the boiler fails
Have you considered replacing the hot water cylinder with an unvented cylinder( you can lose the cold water storage cistern) and a heat only or system boiler, even combi boiler for heating ( combi supplying water to kitchen/ utility). Pressurising the system may reveal weeps in the system so keep it open vented.