Virgin Mobile is now a rebadging of O2, and the rebadging will soon disappear before it's just an O2 account.
Data speed & quality of coverage are dictated by different things. The quality of coverage is what it is, but data speeds are dictated by the amount of capacity on your serving cell, divided by the amount of users there are wanting a piece of it. Nationally, O2 is consistently the worst performing network for data - and this was before Virgin moved their mobile customers from VF to O2, making the situation worse.
You can have poor coverage but quick data just as easily as you can have excellent coverage but poor data. The fix is for more capacity to be added on the radio network (both 4G & 5G), but the ease with which this can be done depends on site-by-site configs. And it's a long-term project, not something that can be done overnight.
As a further example, EE & VF have an active programme to switch the 3G network off and fully re-farm the spectrum onto 4G & 5G services. O2 have not yet announced any dates by which this will be complete.
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