[ap4a]

The 5D doesn't have a built in flash, but it doesn't 'require' a flash. If your main use is for landscapes then you won't necessarily find much use for a flash, except for the occasional fill flash. If you hope to also use it for portraiture then you will need one, however there's no requirement to have it mounted on the camera, and you would get better results with it off-camera anyway - a simple light stand, or second tripod, with the flash mounted would give you good results. However, if your budget is a max of £1,000 then you're not going to get that kind of kit together anyway if the body is going to set you back £800. The 5D will show up all the inadequacies of cheaper lenses and you wouldn't have enough left over for a flash unit.

A 450D (£408) or 1000D (£311) would leave you with enough left over for a Sigma 10-20mm (£350), a Canon 50mm f/1.8 (£82) and a 430 EX II flash (£188) or a 3rd lens such as a 55-200 or 75-300 which while not great will give reasonable results for a bit of nature photography.