This is a question that seems to come up quite often and although we dont stock any large shrubs I can provide some pointers. If you are planting shrubs for bees then, ideally you would plant a few different ones to provide both you and the bees with flowers at different times. Shrubs (and trees) often flower when there is very little else out; early in the season and even some in winter, so they are also very useful to supporting the bees at these times.
March/April: traditional hedging plants are idea - blackthorn flowers first followed by hawthorn. Both prickly but masses of flower.
April/May: fruit trees and bushes - grow dwarf varieties of plum or apple. Both fruiting and ornamental currents are great too.
May-Aug: now you have masses to choose from: escalonia, hebe, roses (simple flowered) buddlea (orange ball type), lavender, hyssop, broom
Aug/Sept: heather and also consider ivy for really late nectar into October
We stock the very excellent bee sub-shrubs of lavender and hyssop.