For the record, I recall writing a post nearly 2 years ago about this topic: https://community.virginmedia.com/t5/Forum-Archive/Password-security/m-p/3902838#M2140843
Apparently Virgin have not done anything about this. Rather bafflingly, this forum seems to have a different password setup and is much more secure than the main Virgin Media account - it allowed me to have a password of length 20.
As a side note, NIST suggests that allowing longer passwords is more important than having more complex passwords: https://auth0.com/blog/dont-pass-on-the-new-nist-password-guidelines/
L0ng-compl3x_p4ssword5~are.b35T^are*4way5=best. Where possible I always use at least 24 bytes.