If that's not a serious comment and you're trying to be smarta$$, then can I suggest that it's time to be quiet .... silly personAnd suppose location and time of year near farmers fields.
The house is 1990's vintage and what I'm anticipating is that the downstairs cold feeds (2 Sinks/Cloakroom Basin/Toilet) are all at mains pressure, but I'll check before ripping anything out.
Can I suggest that a proper design is undertaken - You need to understand what the system will need to cater for, what you expect from it as far a usability is concerned and what the cold mains can deliver - as a starting point.
If you are talking about structural works, new CWSC, and everything else to increase capacities then I would suggest that there may be better alternatives, i.e. unvented, as I suggested before.