Depends how much you're willing to pay for two connections for the whole 30day notice period.
I left VM recently and overlapped the connections, and it was well worth it due to a brief teething issue on my new FTTP connection. Because you still need to give 30 days notice to VM, then if you allow a week of the new connection before serving 30 days notice to VM, that's five weeks or so of paying for duplicate connections. Realistically, a loss of service on an Openreach connection should be resolved in two-three working days, so if the installation date is met, then your calculation is whether five weeks of VM bills is worth paying to avoid a potential three day loss of service? Of course, if the new connection is delayed eg due to blocked ducts or a need to repull your Openreach line, delays could be longer but with an overlap you aren't inconvenienced at all as you haven't started your 30 days.
Personally I'd always recommend overlapping, but I'm in the position that the £60-70 that it cost me was a justifiable and affordable insurance against the wrath of the remainder of the household. A cheaper alternative but with less surety against a delayed install would be to start VM's 30 days notice so that there's two weeks overlap - and usually you can defer a planned VM disconnection during your notice period.
