That is a strange account. I have a HSBC Premier account in Hong Kong, and the UK branch will not allow me to link them together. So I can do that business idea, which someone on this thread had suggested. But I don't have the time to do it, and I also don't want to steal your business. 
Maybe your account is a credit card. I have the premier account too, however I went to their Hq in central and confirmed that I have to pay fee and rates, which is not profitable in the long run.
No my account is a current and saving account. There is no fee in doing currency exchanges as long as it is within your account. This is normal.
When you go back to Hong Kong next time. Look at all the old women sitting around the bank, looking at the exchange rates. They are trading currency. That is why they are there, and because there is no charge when exchange currency within their account. That is why even old women love trading currency in Hong Kong. Also there is no income tax for trading.
In the UK, if you trade currency, the government will take tax from your profit.
There is exchange rate fee, if you don't have that currency and need to purchase. But if you put in say, GBP into your account, then do exchange within the account, there is no fee.
