When I leave VM will I still be able to access those emails.
Once you close your VM broadband account you can continue you access and use any Blueyonder or other Virgin Media email accounts linked to your VM broadband account for 90 days. After that VM can delete the email accounts at any time.
So that means you have a 90 days period of grace after leaving VM but VM will eventually delete the email account and all emails and contacts it contains.
I have a stack of blueyonder emails saved in Outlook that I’d hate to lose.
The emails should still be available in Outlook even after VM close the email accounts. However to be safe you also need to download / backup the emails in Outlook and store then as a file away from Outlook. They will be saved as .pst files which you can always access either via Outlook or via a third party PST viewer.