As I also do Bonsai I'd have said Beech for your 1st photo because of the grey/smooth looking bark... but I'm not seeing any leaves *still attached to the twiggy branches in the 2nd lot of photos uploaded* - beech tend to hold on to (some of) their leaves protecting the buds for the next season... plus the leaf buds are rather staggered going left and right/alternate - quite pronounced... and the bark on the trunks is rather 'thin' compared to something like oak. If that's unclear I'll see if I can take some photos of parts of my beech bonsai trees.
Also your second lot of photos show rougher bark on some... we're a mix of trees brought down - if storm damage? I'd be thinking some Ash based on those other photos. You'll probably not know for certain until you plank them - if your are thinking of doing that?