- would it be cheaper to book from Hong Kong rather than the UK ?
Try zuji.com.hk and compare.
- Northwest shows as the cheapest flight (around £260) using expedia, anyone flown with them on that route ?
From memory this is a codeshare through to SFO and pretty terrible last time I did it. Ancient & rude aircrew
- are there any good, cheap airlines flying direct on that route ?
Not cheap & direct but China Airlines is cheap but goes via Taipei. They have had a colourful history but I guess you get what you pay for and I've always found them very good.