I'm probably not the best person to answer this question, but I'll state how I see your options. The highest grade Canon you can get for this amount is either the 550 or 600D. Both will be a great step up from your 400D. Both have high quality LCD screens, great low light performance & features. The 600D being basically an updated 550D, with a vari-angle screen & more shooting modes. You can get a 600D, body only, for just under £600 online; but if you'd rather buy from an on street retailer, you'll probably only get the 550D for the same price. So my advice would be to go for the 600D over the 550D, but wherever you look to buy it from, don't pay too much more for the 600D over the 550D, it may be better, but it isn't, in my opinion, worth a £100 or so premium.
Good Luck.
P.S Whatever your decision, don't rush on it, It's better to take longer and be truly happy with your choice.