Power Engineering in Uxbridge, west london do a superchips chip...
That sounds like a reasonable deal for a rolling road tune. Depends how long they spend on it on the day, of course, but if they're kitted out to modify the maps on the fly on the rollers and then burn you a chip with the new maps on, that seems reasonable value. It's certainly in the same ballpark to what I would expect to be charged if I turned up with a mappable ECU, although it won't be a complete re-map, just a tweak here and there.
I decided to be difficult and spent the money on a wideband Lambda sensor so I can tune on the road

As to how much difference it makes, I don't know as I've never driven a chipped Omega. As with all natasp engines, there's no magic boost value that can be turned up so you're really limited to improving bits of the OEM map that are lacking. Others have said it makes a worthwhile difference, however.
Kevin