Many major bus operators in UK tend to set shorter service lives to buses than those in HK.
Common service lives of UK buses from major operators are only around 12-14 years (the
extreme case is 7 years in London).
Moreover, some bus types are actually designed for a short life-cycle of around 10 years.
Darts & B6s are some of these examples. Hong Kong operators are using them over the
life-cycle somehow.