The sell by date means nothing once you freeze them, that refers to the fresh meat and when it must be used by. Sure how would freezing stuff work at all if it still only had a few days of a shelf life? The meat Tesco sells cheap is because it's coming to it's sell by/use by date for fresh meat. Totally different ball game once you freeze it.They may not decay but they do pass the sell by date which could lead too all sorts of issues.
That’s the reason the likes of Tesco sell some meat cheap as it’s very close too it’s sell by date.
Different if you buy for example a prefrozen dish, it might (I don't know I've never looked) have a 'use by' date on it but again that will only be related to quality and again freezers differ, some keep stuff good for longer than others.