When you say a "purchased boxset", do you mean a PPV using the Virgin Media Store?
VM OnDemand programmes, whether they are boxsets (inclusive on Maxit top-tier) or chargeable PPV options as I suspect you're asking about, are for viewing on your set-top-box and are not downloadable in the way you hope.
I recall some options in the past, included a posted-DVD copy - but this could depend what you mean by a "purchased boxset"
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