I will have to manually delete them from the VM server.
I really don't envy you deleting 210,000 emails. Going through that number of emails, deciding which to keep and manually deleting the rest could result in physical and mental health issues. 😉
If your main concern is to slim down the inbox to speed up Outlook synchronising with the VM inbox then you could try using the archive function in webmail.
Click on Inbox, then settings icon (3 bars) beside it and select the Archive option. As it says, all emails over 90 days old will be moved to the archive folder. You can always move specific ones back to the Inbox or into another folder if you ever need them.
As has been said though, any emails that are super important need to be archived "properly" to a more secure location.
******************************* I am just another Virgin Media customer. If someone posts a useful reply you can say thanks by clicking on the thumbs up sign in their post. If someone posts a message that solves your problem it helps everyone if you mark their post as a Helpful Answer
As an update to my original post I will outline how I resolved this issue. I will tell you what you need to do, and in what order to do it, but not necessarily how to do each thing. If your not sure google it. There's plenty of help out there or within this forum.
The original reason I used POP3 is because I don't like keeping personal emails on another company's Server. IMHO, one way or another data will get leaked somehow, somewhere eventually. I prefer to keep my data local and have automatic backups to my own RAID server (NAS)
So presumably your existing email address is POP3. As discussed previously, this cannot directly be changed to IMAP. The POP3 email entry must be deleted from Outlook first, but if you do that you'll lose everything, folders email rules etc.
So first, log into your Virgin Media account as the Primary account holder, go to the Profiles heading, then Manage Accounts. Set up a new account. This will be a secondary account and whatever you call it will also be it's email address. However, you're never going to use it for email so call it what you like.
Now go back to Outlook and Add new account. If you let it do this automatically it will become an IMAP account, which you DO NOT want it to be, so make sure you select the option to create manually and set it up as a POP3 account. Any port settings etc are easily found on the web.
Once you have this account in Outlook, you'll need to move all your folder structure from the existing (genuine) email account in Outlook to the new one just created. This is quite quick to do and you can move folders full of sub-folders all at once. Once this is done, if you use Rules for filtering emails you will want to keep them, so Export your Rules to a file. Once you're happy that the original email account is now empty of everything, you can now delete it. Deleting the email account from Outlook will NOT delete the account from Virgin Webmail.
Once deleted, in Outlook, Add new account, and use your normal email address (same as the one just deleted). This time either set up manually as IMAP or let it do it automatically which will also be IMAP.
Once set up, leave the default folder structure (inbox, Sent, Drafts etc) as they are. Use this account only for Receiving/Sending emails, remembering that what's in the Outlook account is also on the Webmail account. They are always kept fully synchronised.
In Outlook, select the new (random name) account and Import the Rules previously saved. These rules will probably need amending so that any incoming emails to your normal email account are redirected and saved to their nominated folder in the "Random name" account. This account is effectively just being used as storage for your folder structure and those folders/emails only exist on your computer.
In your normal incoming emails, if you delete or move emails, the Server doesn't care where they've gone, they now don't exist in your Inbox, so it will remove them from your Webmail (Server) Inbox. Same as if you delete them from your Deleted Items folder, they'll disappear from Webmail too. So ultimate goal achieved. No data kept on server, and emails automatically deleted from Webmail, but total folder structure/emails still remains in Outlook.
As an added bonus, I set my mobile phone email app to also be IMAP and when I delete/move emails in PC Outlook they automatically disappear from Mobile Phone as well.