For £1k there are plenty of options. The one I’ve had most experience with (and thoroughly recommend) would be the Canon 70-200mm f4 IS L.
It’s light and small (for the quality if offers), is pin sharp, has excellent IS and is very highly regarded amongst all that use it.
On your 50D it’s a 112 – 320mm equivalent, which should suit most needs, but if you shop around you may squeeze the 1.4 extender to give you 448mm at the long end in under your budget.
Cheers,
Steve
ImageArcade – Photography By Steve Ayres