The best deals seem to be achieved by phoning and cancelling then awaiting a call from the outbound retentions (loyalty) team. Just thinking of leaving will likely get a small reduction, but from reviewing various discussions online most people get the best savings by actually cancelling. Make sure you have your account settings (check online) so they are allowed to contact you. This seems to be how they have designed their system (lower discounts to appease mildly disgruntled customers, bigger discounts for those actually prepared to leave), so use it.
Some of the recent discussions and deals achieved around this are captured on the following forum (as an example):
https://www.hotukdeals.com/deals/vir...r-31st-3017287