I agree with Just another number- you get a much better exchange rate for the Hong Kong Dollar in Hong Kong itself (I'm currently in Hong Kong). Yesterdays intra-bank exchange rate was $12.26HKD to the pound. I managed to get $12.22HKD. The buying rate was $12.35HKD so the spreads can be tiny.
Chungking Mansions has some of the best rates in town. It also has some of the worst so make sure you shop around. World Wide House in Central has some good rates too (it's always full of Filipino domestic workers wiring money home).
I also echo Just another numbers sentiments about the US dollar. USD/HKD are pegged so there is vary little variation in the exchange rate you get on the ground here.