I would first decide whether you want to go down the superzoom or DSLR route. If superzoom you're probably looking at choosing between Panasonic and Canon and £400 will get you everything you need.
For wildlife photography £400 is quite limiting but we all have to start somewhere.
Always been told (and believe this a very important point) that having the top glass is better than having the top body.
So I would maybe try and get a Canon 30D for around the £150 area then save for the Canon 400mm f5.6 or similar lens, the sigma 120-400mm lens is very capable and a lot cheaper so worth a look too.
That said superzoom/bridge cameras will give you all of this for within your budget so it just depends how far you want to take it
Lots of helpful people here so ask away!
Is the Canon 400mm f5.6 you also mentioned, a separate lens that can be bought separately?
A used Rebel or XXXD series goes for even cheaper than the used 30D, but their capabilities are somewhat more limited. Nonetheless, I've seen good shots until today shot from these purportedly 'obsolete' bodies.
As for the lens, the cheapest prime lenses would be used Tokina or Sigma 400mm 5.6s. Some eBay links here:
http://www.ebay.co.uk/itm/Canon-EOS...=UK_Lenses_Filters_Lenses&hash=item27d0b1a8ce
Note that they may or may not AF with the Canon bodies. If you do decide to go down that route, better to buy used Nikon bodies like the D70 or something like that (with an in-body AF motor) as generally these third-party lenses maintain compatibility with older Nikon bodies in a much better way compared to Canon. That way you could squeeze a DSLR and a long prime lens into your budget.
The 650D compared to the 30D, also built-in speedlite transmitter, 63 zone metering, video, has DIGIC V & 14bit image processor compared to the DIGIC 2 & 12bit of the former. The 650d also has Live View LCD, useful for many other wildlife projects. I wouldn't buy a camera without LV anymore so I would strongly disagree that the the 650D would be limited in capability, considering it also has more than twice the amount of pixels which would help in post-processing & cropping.