The United Kingdom isnt a country, England, Scotland, Wales and Northern Ireland make up the UK and a few other places but we wont go into that. Sorry for sounding like a twat but just thought id let you know.
I've not looked up any stats but i'd be surprised if this were the case. Think most sales would likely to be a European country possibly even the UK given our irrational love of convertables. BMW's aren't massively common in the USA given their relative cost compared to US-made cars.
UK has the highest per-capita incidence of convertibles, but that doesn't mean we're the biggest consumers. Take for example the MX-5 which is a 'girls car' and probably one of the smallest cars on sale in the US. In 2006 16,897 MX-5s were sold in the US; meanwhile 22,546 were sold globally (inc the US?): http://en.wikipedia.org/wiki/Mazda_MX-5. If the MX-5 sold best in the US, imagine a US-centric convertible with a BMW badge...