Water flow rates for vented cylinder

Hi all,
At the start of the process of getting a heat pump installed so good opportunity to switch to an unvented system. Currently hot water is from a vented system and both hot and cold water are gravity fed (cold water tank in loft).
The installer engineer took one look at the mains supply and said it was 22mm ish plastic so probably equivalent to 15mm copper and hence the flow rate wouldn't be enough to go unvented.
I've measured the flow at the kitchen tap (only mains tap aside from tank fill valve, looks like 15mm copper feed) and it does about 17.5l/min but haven't got a pressure tester to test the dynamic pressure. I had a quote from BG for a new boiler and he had a proper tester which showed the flow was good enough for a combi (IIRC which I may not be, was a while ago) if I wanted but don't remember the figures.
It'd be good to get rid of the shower pump, have decent pressure at all taps and reclaim a huge chunk of loft space but wouldn't want nasty surprises when taking a shower as someone else turns a cold tap on :)
Thoughts? What flow rate would be needed practically to have an unvented cylinder?
Thanks,
Peter.
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