Jonathan,
there are quite a few to choose from as I am finding out. I am about to try to sell me Myford ML7 and upgrade to something decent.
A Boxford BUD in good condition with accessories should be in or around your £800 limit. You might be lucky and get an AUD model with a screw cutting gearbox for that price. A Raglan 5 inch would also be a good choice if you can find one. AColchester Bantam 600 weighs in around the 400KG mark and would als be in your price range. This a very good bit of kit. Smart and Brown Sabel if you can find one. There are some good machines on ebay just now but unfortunately for me they are all in England. You can check out www.lathes.co.uk Very good site with loads of info on machines.

Regards

Ian