Not good dude. This could have been an easy complaint for them to sort out and get off the system but they are choosing to make it worse. Coronavirus isn't an excuse, if there is increased demand or reduced staffing levels to the extent that they know they can't respond to complaints or queries in the normal time window then the letter should have reflected that and said 14 days (for example). Depending how strongly you feel about it you can take it further. To be fair though you aren't going to get anywhere just yet as although they said they would get back to you within 7 days, the rules say they have got 28 days.
https://www.virginmedia.com/content/...April-2020.pdf
If I were you I would get a piece of paper and start making a note of dates of when the first phone call was, when you rang back to query it, when you sent the letter they acknowledged receipt of etc etc so you have got a clear timeline of events.