Most flowering shrubs are useful to bees; unlike a lot of herbacious perennials, it seems that plant breeders have not mucked about with shrubs quite as much and so very few of them have sacrificed pollen and nectar for beauty. The best shrubs for bees are those that will produce a mass of flower that will really create a major bee-attraction but you must be aware that where you plant it really matters; bees greatly prefer to forage in sun therefore, the bee-effect of a great shrub will be wasted if you plant it somewhere shady.
Top bee shrubs:
Native hedging such as hawthorn and blackthorn - March/April - great for pollen and nectar before perennials start flowering
Single flowered roses such as our native ones (also see David Austens selection)
Fruit trees - dwarf apple, pear or quince trees are a great alternative shrub or gooseberries - and provide you with fruit too! Blackberry is also a real favourite if you have a garden big enough for the brute.
Flowering currants (ribes) - April/May flowering
Hebes - popular with bumblebee
Lavatera - August/Sept
Herbal sub-shrubs such as lavender, sage and hyssop are all fab and provide essential oils as well as nectar. These will grow to c. 60cm in a sunny spot
Buddleia globosa (orange ball flowers) not the purple one although that is great for butterflies
Cotoneaster - can really buzz when in full flower
Ceanothus - various species flower either spring or autumn