If a hacker gets access to your email, they will use it to hack other accounts as you mention such instagram, facebook etc. They do that to get as much information about the victim as possible, and may be for fraudulent purposes or identity theft.
As the virgin media email does not have 2 factor authentication (2FA) hackers only need a password to get into VM email. I do use a strong password with 20 digits for my VM account/email which make it harder and less likely to be hacked and change the password often, I do that regularly every few months or so.
Wise to change your recovery email(s) on any accounts linked to a hacked email, replace with a gmail.com or outlook.com email and set up 2FA. which links to a mobile phone number(s)
If accounts are compromised, try to recover them, I had a similar event with facebook in the past when I was locked out in 2020 and had to prove my identity.
If no longer a customer VM can delete out a compromised legacy email, but if still a customer it should be possible to recover the account yourself or with a forum member and then change the password and security question once access is gained, a primary email address can not be removed if the VM account is still active, I have an old ntlworld.com email still active and pwned so gets spam.
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Note any legacy email should be not be active after 90 days, so odd so many old emails seem to be hacked.
The email password is the same as the VM account password.
I use outlook.com to log in to my VM account and find that more secure as it has 2FA, and that gives access also to the VM email page.
I no longer use the VM email to link to any of my accounts such as banks, online shopping, facebook, instagram, twitter etc and have switched to other email providers now.
I have noticed the VM security captcha robot check only appears very rarely now, it used to do a robot check every time I logged in , but not now?