Verify it's you by changing your email address

I'm currently very confused.  I'm old so it might be me but I think it isn't.  

I couldn't log into my Virgin account via Android/Opera on my phone this afternoon so I decided to try on the pc.  Windows 10 Opera browser if it's relevent.

Logged in fine.

Noticed that there was a long defunct mobile number set as contact phone so thought I'd change it to my Virgin Media landline.

On trying to do this VM told me I have to verify myself by changing my email address to a non Virgin email address which I would use to log in with in future.  They also said I would use this non Virgin email to log into my Virgin emails.

Is this confusing?  I'm confused anyway.  I use Gmail app on my phone to pick up my Virgin emails.  How is say a Gmail email address going to log into a Virgin email address on Gmail app?

Having logged into here to ask I find a thread saying Virgin have told them to change back to logging in to their account with their Virgin email address instead of their non Virgin email.  Which is the opposite to what Virgin wants me to do to verify myself to change a phone number.

I'm reluctant to change anything because although I only use my Virgin email for Virgin things ... and it auto forwards also into a different non Virgin non Gmail account, being elderly I like to keep things as simple as possible because ... well stress ... who needs it. 

I would love if someone would explain how I can only verify myself by adding a totally unknown to Virgin non Virgin email etc etc for starters.   I'd also love to know how I'm going to pick my emails up via Gmail app if I'm using a Gmail address to log into Virgin email.

For now I have backed sadly away from the verify it's you page ... because it doesn't make any sense at all.  It would make more sense if they used the auto forward email they already have on the account to verify it's me ... but changing my sign in to something unknown doesn't verify it's me - it could be anyone ... perfect way for someone to hack the account really ... 

Or maybe I'm just not getting it.  I don't know.  If this could be picked up by someone from VM at some point and explain that would be nice.  For now since the current set up has worked since 2007 I'm loathe to change it.