I don't use any Apple or ios devices but for what it's worth here are my thoughts based on what you have posted:
"I updated my password. This enabled me to regain access to my mail account via WebMail and via IMAP on my computer."
So it seems that currently there is no inherent problem with your VM email account itself or with the Virgin Media's IMAP server. If there was a problem with either of those you would not be able to access your emails via Webmail and the email app on your computer.
"unable to update the email password in my iPhone email account settings. When I attempt to do so, I am informed that my blueyonder email address is invalid"
So if the VM email account works on other devices and email apps that suggests the problem you are seeing is caused by something in the iPhone Mail app itself.
As I say, I'm not an ios user but I have seen this complaint come up before on the Apple community forum. My advice would be to try deleting your VM email account from the Mail app (not from the VM website!) and then try adding it to the app as a "new" account.
That worked for this poster on the Apple Community forum https://discussions.apple.com/thread/252894673
They were trying to update their email password for an account in their Mail app but continually getting the "Invalid email address" error. When they deleted the account from the app and then set it up again it all worked fine without a hitch.
The "delete and set it up again" trick is always worth trying when you have a problem with any email account any email app.😉
Try it and see how it goes.
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