You keep your boosted broadband speed if you leave o2. I changed to ID mobile a year ago and I still get 500Mb for the price of 350Mb.
This is also stated on the Virgin Media site in the help section.
Quote:
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If you leave O2, you’ll lose your mobile benefits, but you’ll keep your boosted Virgin Media broadband speeds. Likewise, if you leave Virgin Media, everyone in the household on an eligible O2 plan will keep their double mobile data until you upgrade – then, your mobile benefits will stop.
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https://www.virginmedia.com/help/virgin-o2-benefits