https://ee.co.uk/broadband/switch
Using OTS can be helpful in so far as you don't have to deal with VM and you may be able to truncate the 30 day notice period to VM. EE should deal with VM for you.
When picking your switch date, you need to complete your minimum term with VM to avoid early disconnection fees (although the link above mentions an offer of up to £300 to cover EDF).
A possible disadvantage of OTS is that you are relying on everything happening, as planned, on the switchover date without any issues. Those working from home, and who need an internet connection, may find this a bit risky should everything not go as planned.
A frequent recommendation on here, if you want a guaranteed continuity of service, is to 'manually' arrange the switchover yourself and overlap old and new services so you can try the new EE connection in the 14 day cooling off period and still have VM to fall back on if any problems with EE arise. Once the new service is up and running as you want, you could then 'manually' give 30 days notice to VM. The obvious disadvantage to this approach is having to pay two subscriptions to two providers during the overlap period and also endure the unhappy experience of doing the cancelling yourself and having to try to contact VM CS.