I'll be the first to admit I've never got my head round TV-Mix vs TV-Mixit, except to say Mixit is the basic package (vaguely comparable to Freeview) in either case.
However, a deadlock letter is only issued once you have exhausted VM's internal complaints process - precisely to avoid complaints going to external bodies that have no need to. Your first port of call should be C/S on 150 (or 0345 4541111).
If you have copies of the paperwork, offer or contract, then just giving VM a chance to resolve this issue would be the best way to sort it.
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