Can you try some boxset content from a totally different broadcaster, perhaps help ID whether this is an issue with particular programs, or all boxsets?
Home > OnDemand > BoxSets > take your pick.
Perhaps try 2 or 3 to help prove the issue.
The Starzplay issue is another thing entirely - that's just VM listing Starzplay content among Boxsets as a tempter to encourage you to subscribe to Starzplay. Amazon & Netflix stuff isn't listed in there, so I personally don't see why Starzplay should be either. By all means list it in the EPG, but I agree it's not a "boxset" and shouldn't be in there.
Both of which are fine for me, which I guess proves it's not a national fault and is therefore either regional specific to you (and I dare say, others) or is something to do with the package flags on your account.
This'll obviously need Virgin support to intervene.