So 'best' in this query means 'cheaper than £350', presumably.

There's an alternative which will probably take two days instead of two hours: Overnight luxury bus to Guilin. Connect with Beijing to Hanoi train at Guilin (runs twice a week). Train passes through Guilin in the morning and arrives Nanning early evening. Passengers get long break to go into town for dinner, then rejoin train for trip to border formalities which occur on the train very late at night, arriving at the Vietnamese side in the very small hours of the morning for their formalities and a change of train, arriving in Hanoi later that morning. Of course, if your destination is Ho Chi Minh, then you've still some distance to travel.

If the overnight bus is not running to time with the train, then you could consider crossing the border to Shenzhen and flying to Nanning and getting a local train or bus to the border and bus or train from there. If you do the whole thing by train then you need first to get to Guangzhou East by train from HK then transfer to Guangzhou Station and take a train to Guilin or Nanning and continue as above. (Or there are bus or ferry-and-bus options from HK/Kowloon/HK Airport to central Guangzhou).

But I'd be very surprised if you couldn't find a better price than that in Hong Kong in person. I don't remember what I paid, I must admit, and time of year and other factors play a huge part, but I don't recall paying anything near that.

Peter N-H
http://members.axion.net/~pnh/China.html