AitchBEE before you use your image check again with Virgin as to what Lotvic stated and ask why you were not notified. I would.

You should only incur roaming charges if you leave the UK.

It's not just you getting the popup message, see this post on CaravanClub (in UK)

"...Orange network kicks in- Virgin says no extra cost, although my dongle comes up with a message saying that 'roaming' costs more- looking forward to my next monthly bill with trepidation in case 'roaming' does cost, even though Virgin says it doesn't"

If you phone virgin up they can remove the roaming message as on other posts they are still integrating two systems & even disconnecting some transmitters where there was overlap so you may now be using a different mobile transmitter than you were - Phone Virgin up & explain your problem.

Virgin piggy back on T-Mobile's network.

As T-Mobile & Orange are one & the same customer's of T-Mobile & Orange can use either companies network, automatically switching between the two without incurring roaming charges.

I'm not sure the same applies to Virgin customers using Orange network, so I'd check this out with Virgin ASAP.

btw - roaming doesn't just mean using networks abroad with a UK mobile, it also applies to UK mobiles using someone else's network in the UK.

If you need to make a call and can't get on your own network (a common problem in the countryside) you can set you phone to hop onto any available network, but you will usually be charged a pretty penny for doing so...