UPDATE 1-Moody's downgrade adds to investor swing away from pound

* Sterling hits 2-1/2 year low vs dollar after downgrade * QE prospects help gilts recover from big initial fall * Markets see risk of further sterling and gilt weakness By David Milliken LONDON, Feb 25 (Reuters) - Britain's loss of its top credit ranking from Moody's added to the pound's weakness on Monday, helping send it lower against both the dollar and euro, but UK bonds, underpinned by the central bank, recovered quickly. The pound only fell moderately, but still hit