Scott, flying through LHR, it may seem like a great number of Golds exist, but in reality, there are not that many out there. In one large European country that I know of there are 1200, in another there are 1500, in another there are just 200 (and remember these guys have really low thresholds). I believe that I read that HKG has the greatest density with about 2500 in the territory. In short, 50,000 seems reasonable give or less a few thousand.
Comparing to the US would be interesting as it would seem that the top three carriers each have about 30-40,000 in their top tiers and maybe 100-200,000 each in their mid tiers. Each of these carriers carries about 3-4 times as many passengers per year as does BA, although their programmes are mile rather than fare based. Furthermore, their levels are generally the same throughout the World while outside the US and UK BA severely deiscounts the amount of flying to get to Silver and Gold (Hell, one connecting "A" fare through London from certain countries can net someone Gold)