I'm travelling from the UK to Changsha International Airport and then onto Huaihua. Would you suggest bus, train or plane and how would I go about it?

Here are two ways for you:
1. take shuttle bus to Changsha South Bus Station, where there are direct bus to Huaihua.
2. take shuttle bus to Changsha Railway Station, where you can easily catch a train to your destination within 7h or a little longer.

Thank you for that. But my location has changed now. I'm now going to Chenzhou. Can you help me?

The best and the fastest method is to take the direct flights. The flights take about an hour and forty-five minutes.
Or you take the trains to reach Chengdu from Changsha Railway Station. The trains take about 19 to 22 hours. There are three trains between: K502/K503, K577/K580 and K787/K786.