As others have pointed out, the theoretical prices in each exchange rate are identical.
However, the cost to you may vary. You need to work out the "opportunity cost" of using each of the three currencies, because you lose money every time you convert between two currencies.
Let's supposed for example that you live in Canada and earn and spend CAD there, but also happen to have some HKD. To keep this simple I'll ignore the USD, but you can apply the same reasoning to that.
If you're not planning on visiting Hong Kong for a while, then your holdings of HKD are just gathering dust, so your choices are to convert them to CAD and make the GBP purchase in CAD, or to spend the HKD directly on the GBP purchase. Probably the first option will end up costing you more in conversion fees than the second, but you need to look at the precise costs to verify this.
If you are planning on visiting Hong Kong soon and will need the HKD, then the choices are somewhat different. You can either spend CAD on the GBP purchase, or you can spend the HKD on the GBP purchase and then soon convert some CAD into more HKD for your trip. In this case the first option will probably end up costing you less in conversion fees.