No-one's going to force you to take any service that you don't want, but you may well find that a bundle with an O2 SIM is cheaper than the equivalent one without - in other words, multi-bundling discounts more than compensate for the additional cost of a given service.
The complication with VM's mobile setup currently is that the billing is split across Virgin & O2. That aside, you can simply ask the retentions agent to quote for your combination of services without the O2 SIM. If that is more than the price with, then you have a choice to make.
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