I don't have anything to add about GBP or AUD, but until recently I kept any money I had in my mortgage linked savings plan. My last mortgage was 2.15% and now my mortgage rate is 2.35%. 2.35% is a decent return for money that's very liquid, but for long term money (10+ years) I think a US and/or Global total stock market low cost (0-0.06%) fund (ETF or mutual fund) is the best way to go, but many smart people say to diversify with a bond fund up to a ratio that matches your risk tolerance.
I think Schwab is great for US Citizens, but others are good too. With Schwab I send money to their Citibank N.A. HK account, and for a super-low total fee they'll convert it to USD and put it into my Schwab brokerage account within one business day. I first use HSBC to transfer HKD to Schwab's Citibank N.A. HKD account for free. This method is cheaper than anything else I've found, and I hate fees.
Good luck figuring out what works best for you.