"I can't imagine many societies in the distant past were ever able to eat a lot of animal protein day after day after day. Hunter gatherers would have had some meat after killing a biggish animal but most of the time they would be living on plant matter (nuts, seeds and fruits in good times, roots, stems, leaves and shoots for everyday, bark and woody bits in very hard times) supplemented with insects and occasional finds/kills of eggs, fish, honey and tiny mammals/reptiles/amphibians/birds. We'd have been scavenging as much as hunting a lot of the time."
Gaspode, I am afraid I disagree.
I don't think people realise just how much food a typical livestock animal contains because we have become so removed from animal husbandry and butchery. Take a hog, which would have once been a wild boar: you can get enough muscle meat off a medium to large-ish hog to provide two "deck of cards" portions for 200 to 250 people. And that is just muscle meat, there's also the fat, skin, organs, blood and bones. All in all, I would estimate an adult wild boar could feed 200 to 250 people about 500 to 800 calories each, if they ate nose to tail, including all the fat, skin, blood, organs and bone marrow.
So if your everyday HG group consisted of about 100 people, with maybe 30 percent children, one wild boar is food for everyone for at least a day.
Hunting an adult boar of average size would not have been a particularly significant feat, particularly considering the overwhelmingly forested nature of North Western Europe at the time that was full of animals that no longer inhabit this part of the world, and the fact you would have somewhere in the region of 30 to 50 adult males in your group of 100. You are also forgetting trapping, and there is evidence to suggest that part of the "gathering" role involved making and setting traps, and retrieving caught animals: small as well as medium, particularly if you use pit traps.
We are talking about a lot of available meat here. And, in Spring, if you hunt or trap unreared baby animals, you are also talking about the availability of a limited amount of cheese (people forget what cheese actually is). And I haven't even talked about fish or birds yet.
The one thing that would have been more difficult in North Western Europe would have been plant matter, small animals and insects during winter. What's interesting in this regard is the recent find at Stonehenge where they have found evidence of large-scale animal consumption (some from large animals that appear to have been reared in Scotland and brought down to the site), but no sign of large-scale plant consumption, which I think is very telling.
Again, I think people forget just how much plant matter you have to eat to get anywhere near 1000 calories. You would be looking at over 4kg of spinach, for example. We have become very desensitised to this reality because of our food culture of processed grains.