Hang Seng bank charge commission even if you have an account with them. They only don't charge Priority Banking customers.
Bank of China(HK) don't charge commission if you have an account with them. But not all branches have foreign exchange service, you have to ask them which branch has.
尖沙咀及機場都有找換店. 佢地會同你對換 ka la. $20 英鎊可換回約 $500 港幣. 去找換店換會少 D lor. 因佢地個對換率通常低 D ka ma. 去銀行換就多番 $20 咁上下 la. 如你有匯豐銀行戶口, 對換唔使手續費 ka. 或者求其搵個 friend 既紅薄仔去匯豐銀行換 lor. show 一 show 本薄或信用卡 ka jar ma.