Well, a 50in screen at 3m might be a bit close for comfort - we normally recommend a screen size of 36-42in for a viewing distance of 3.5-4m, and 28-32in for a distance of 2.5-3.5m. It depends to some extent what you'll be watching on it - HD material at that distance might look OK, but standard-def broadcasts could look a bit ropey.
Both Samsung and Panasonic make an excellent range of plasma and LCD screens. You should look at the Samsung LE32R87BD (£700), Panasonic TX-32LXD&00 (£1000), Panasonic TX-37PX70 (£1000) and Panasonic TH-42PX70 (£1000). They should all be available for quite a bit less in the January sales.