T-Mobile isn't a network in the UK anymore. What was T-Mobile is now part of EE (and are owned by BT).
So if you still have a Virgin Mobile PAYG SIM, it will stop working soon, but if the phone is unlocked you can put whatever SIM you like in it.
If it's an EE SIM, then it won't be affected by the closure of Virgin Mobile PAYG.
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I work for Virgin Media - but all opinions posted here are my own