I think Castrol/BP might make the Quantum, but don't assume it's as good as one of their own oils. All of the own brand oils are made by one of the big manufacturers to a reduced budget. It means the oil is fairly cheap (sometimes), but it's also lower quality than the branded stuff.
505.00 isn't a massively hard spec to meet. It has to be a decent oil, but not as fussy as things like the 507.00. If you want the best cold start protection, use a 5w-40 ester based synthetic. Although the 0w-40 Mobil will flow a little quicker, the ester content of the top 5w-40 oils means that there is already a layer of oil in place, so it's protected from the moment the engine starts working (like in the Castrol Magnatec adverts, but it's a lot better at it than the Castrol). The one's I'd consider are the Fuchs Pro S, Motul 300V, Redline, Millers CFS and Gulf Competition.
http://www.opieoils.co.uk/c-656-5w-40-engine-oil.aspx
Guy, the other Oilman on here, uses the Pro S in his PD engined Leon (which is harder on the oil than the non-pd diesels) and the other oils I mentioned are on a par with the Pro S. At the moment, with the price of the Gulf, I'd go for that. Although those oils do not officially meet the VW specs, unofficially they far exceed 505.00 and 505.01.
Cheers
Tim