If it's like an orchestral piece , I think it's called Requiem For A Tower by Corner Stone Cues. I haven't actually seen the bit where the judges come on, but I know this tune is used sometimes on that show. You can buy it on iTunes.
A lot of the music used in Britain's Got Talent came from a film called Requiem For A Dream. The bit of music in particular I think you're looking for is called Lux Aeterna. There are many versions available from download sites like iTunes etc.