First of all, it's not strictly speaking an ad - it's a channel ident. Though admittedly it does *look* like an ad, which is one reason why a lot of people (myself included) don't like it very much! There are currently eight idents in use on BBC ONE (as it's strictly called now) - the one you're referring to is titled "Festival". If you listen carefully, you'll notice that all eight use different arrangements of the same tune; it's quite hard to recognise on the slower ones, but easy to spot on the upbeat versions like Salsa (all those couples dancing in a big hall) and Tapdogs (tap dance troupe larking about in a disused power station). The music was composed specially for the BBC by Peter Lawler, and there are no plans to release it commercially.

Here are some links to check out: http://www.thetvroom.com/p-bbc-one-2002-a.shtml gives a lot of info on the current idents including details of where they were filmed and the names of some of the people featured in them; http://www.tvhome.co.uk/bbc_one.shtml has nice images of the current idents; and http://www.bbc.co.uk/bbcone has extended versions of some of them to view online.

It sounds a lot like "State One - Forever And A Day" Enjoy