Whether the thief gains from the act may well be immaterial, but surely it does matter that somebody must lose out for it to be theft?

If Tesco had already written the value of the goods down to £0 in an official ledger of accounts, which is what I understand all supermarkets do before they bag it up for disposal, then I would query it's legal status as property that could be stolen? The only sense in which it remains property, from my perspective, is that Tesco remains liable for its safe disposal.

There may technically be a crime at some level here, but doesn't it rate somewhere below that of a man who, upon seeing a penny in the street, stops to pick it up (the value of the goods he has illegitimately seized being greater than zero)?

Agreed. Although I'm not at all sure Tesco wants to stand up in court and say that they wanted to go through a load of bin bags and retrieve formerly frozen goods to put them back on sale.