Can you remember for certain when your BT service started or when you last accepted a new minimum term, TBH if the early termination charges are £140 that seems more than the charges that would apply if it were just a discrepancy between 14 day notice for regular migrations and 30 days if quitting completely.
If you asked to port the phone number and VM used the correct process ( VM giving your notice to BT), and this does seem to have been done as you got a 'sorry to see you go' letter, if you were not bothered about the phone number and didn't ask for a port VM probably wouldn't inform BT , you would have to do that yourself, but that doesn't seem to be an issue.
The VM network is a completely separate one, so if you got a new number , the BT service would stay until you told them to stop it, VM asking for the number stops the service as you cannot have a BT line without a number on it, once the number goes so to does the account the number was associated with.
It could be what you think of as your end of minimum term date ( not end of contract ) is wrong and it's a lot later than 25th Jan, if you haven't already done so, I would check what date your ETC's are based on, and check against any emails from BT welcoming you, or advising you of any new 'deal' and any 'new' contractual dates, it could be you thought you had a 12 month term but it was 18 , or you accepted an offer from BT that gave you a new minimum term later than 25 Jan