Try at 8am is generally reckoned to be a good idea. Or when lines open on Sunday morning is often a very good time.
A higher risk strategy, cancel and hope for an outbound "retention call back". If you get the call this will probably be the best price, if you miss the call (or it is never made) then you've cancelled and either services cease, or you have to make a rather embarrassing call explaining that you didn't mean to leave, and can you retract your cancellation. If Openreach speeds would be acceptable, you could try and schedule a connection for the day before your VM services are formally cancelled (noting the risk of any delay on that new connection), order a Sky or BT package, and then you're covered for continuity, and you've got a very strong negotiating position if that retention call does come.
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