Common box or Buxus sempervirens is very common in UK gardens as a hedging plant, particularly for smaller hedges where it can be easily and quickly clipped into a dense mass of foliage.
There is some doubt as to whether it is a true native of this country and it may only have been truly wild in southern England. It is naturalised though and that in itself shows how happy the plant is with UK conditions which in itself makes it very easy to grow and to keep as a bonsai subject. The natural growth habit lends itself to bonsai style more along the lines of the Chinese penjing or clip and grow styles, in effect creating clouds of dense foliage rather than the more conventional defined branch structure images we may be used to.
There is the opportunity to go either way with this example I think in that a branch structure is well defined and this can be maintained and improved upon with careful selective pruning, or you could simple allow the foliage areas to become increasingly dense simply by clipping to shape, the choice is yours. The tree is 68cm. high with branching 51cm, across. The diam. of the trunk at soil level is 6.5cm. It is currently planted in a plastic bonsai training pot 38x26x10cm. but might benefit from being transferred into a glazed ceramic next spring.