I never thought I'd buy their meat, but a lot of it is British and of excellent quality normally.
I'll comment on the poultry side of things here, seeing as I work in a poultry processing factory ....
Chicken wise, the factory I work at supplies Tesco, Asda, Morrisons, Sainsburys, Co-op, Aldi and Lidl, plus various wholesale outlets. The chickens used for the Co-op Extra Tasty cook in a bag (for example) could easily be used for either Lidl or Aldi, as they all use the same specs. It's just the coating used varies slightly (depending on when the supermarket wants).
It's the same for Tesco's Extra Large chickens. The same birds could easily be used for Morrisons, Sainsburys or Asda, as once again they all have a very similar spec.
It's not just the whole birds either, as we also do ready meals and various portions for the same customers. The main difference is the packaging, but ready meals have very similar specs from all the supermarkets. Again the only difference is the other ingredients used (such as the stuffing used in some meals) differ slightly according to the specifications supplied by our customers, IE, the supermarkets.
On a further note, most poultry supplied to the supermarkets tends to originate at something like 8-10 different factories nationally, of which at least 6 are owned by the same company.
Anyway, don't believe the mantra that the discount supermarkets are demanding better quality than the 'big 4', becasue, as far as I'm concerned, from the factory I work at at least, our customers all get equally good chickens and ready meals.