I am writing this on behalf of my Dad who is 91 and watches a lot of sport. The BT Sport HD channels and FreeSport channels constantly pixellate or have flashing colour bands across them. I have unplugged everything, re-cabled and re-booted box, but to no avail. He does not record, just watches live TV, most of the other channels he watches seem to be OK.
Just by means of an experiment, see if your Dad has any problems with, say, Nat Geo Wild HD, Sky Nature HD or Sky One HD.
Otherwise.. you've done everything necessary to prove this is a fault (either with your Dad's box or his cabling) which Virgin need to rectify. The fault is causing poor TV signal and this causes the pixellation you describe.
The quickest way is for him to call VM TV Faults on 150 (or 0345 4541111 from any other phone) and follow the options for "I have a fault with my TV service" - if it would help, I'm sure he can then pass the phone (after passing security) to a nominated friend or relative (I don't know where your Dad lives, as far as Covid restrictions go?) who can speak to C/S on his behalf.
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