DPA stands for Data Protection Act, and in this case it means that you'd known enough details to pass the relevant data security checks on the account. Unless you have a power of attorney agreement, or are a named individual on the account, that should also mean that you have the account holder on the phone to confirm that you have authority to negotiate on their behalf. Bear in mind that because of VM's woeful customer service, any call to change a package could be repeatedly passed round different teams, and each time you should have to go through the ID checks.
Before going any further, get yourself over to USwitch, and see what comparable packages are being offered by Sky and BT, and how much they are. In negotiating a discount many people don't realise that customer loyalty counts for nothing, value for money counts for nothing, "fairness" counts for nothing, the deals VM offer to new customers count for nothing. The only leverage a customer has is the threat of leaving + knowledge of better value equivalent deals on offer from competitors.
You need to move fast if any changes have already been made to the bundle or price, then the 14 day cooling off period has started. Once outside that they're locked in for 18 months. I'd recommend against either the online chat or the online upgrades - only the "customer retention" team can offer VM's best deals. Having done your research at Uswitch (or any other price comparison website), phone up selecting options for cancelling the VM service, know before you start of the call what you want the price/bundle outcome to be, and know what you will settle for (have these two boundaries written down in front of you along with the table above). You need to have these boundary values firmly fixed because you won't usually be able to negotiate a deal and leave it on the table whilst you consider it, that's not how the system works. The agent you are dealing with will probably be paid a bonus if they can retain you, but won't be paid the bonus if you end the call with "I'd like to think about that". Even if there's notes on the system, any subsequent agent will probably want to start the whole discussion from scratch.
Explain that the current package is still far too expensive despite the recent discussions, you're invoking your cooling off rights with a view to leaving VM, and you hope they can help because you're seriously considering the offer from Company Z, and that's only £Y a month, and would save you £XXX over 18 months. See what they say and what discount they can offer. Usually the first offer is far from the best, and it that applies then say "thanks but that's still not enough, it is nowhere near Company Z's price, starting from the price you just quoted, what does that become if I downgrade my package?" Ignore any "throw ins" that you don't want such as (perhaps) a free SIM or a channel package you wouldn't have chosen.
If you're going to accept the VM offer, this will be a new 18 month contract, get the agent to run through all important aspects including speed, any TV bundles (and channels important to you), any landline and landline add ons, so that this should be recorded on VM's systems in case there's any problem later on. If the BT or Sky alternatives are adequate, and VM's best offer still above your minimum, be prepared to say "no" and walk away and cancel. Given that it seems the 14 day cooling off period has started, you may need to cancel the changes under cooling off rights, but not cancel the service outright until the new service is installed in order to make sure they are not left without service. Make it clear that's what you are doing to the agent if it comes to this.
