A native crab apple would be good in a large tub and would limit the size of it - I've been thinking of putting one in a half whisky barrel for some time. Crab apples support a large variety of wildlife with insects going for the flowers and birds for the apples. You also have the bonus of taking some apples for making apple jelly! There are one or two more ornamental varieities that whilst are probably not as good for allround wildlife are still very beneficial. Japanese Crab (
Malus Floribunda) is covered in pink flowers in spring which insects are supposed to love and very small fruits follow that are taken by birds.
The Silver Birch is a beautiful native tree that whilst too big for a pot is easy enough to limit in size. My two aren't over 7 metres, yet are lovely mature specimens and I live in an end of terrace Wimpy estate house so it is possible.
Rowans are also lovely trees with plenty of wildlife benefit. I believe there are some smaller varieties that may suit a pot. I'd have a look on the RHS plantfinder for you but its currently down
. Here is one such possibility listed on the Shoot Gardening website:
Sorbus reducta - shoot
Hope this helps.
Graeme