If that was my car I would only run the recommended plug in it. Seems like your parts store needs to swap plugs with you. I have found that buying parts from MINI shops pays dividends. The local parts place know less than zero about these cars.
Bosch is a good plug and I have used them many times in other cars, but like the rest I only use NGK in my mini. When I went to get plugs for my 02 the guy at the parts store told me that the type of NGK I wanted was only for motorcycles. If the thread and depth is the same it wont hurt you car they just might not last as long or are note the proper heat plug. Here is a good article about spark plugs and it list the correct ones.
And, just a FYI, if you own a second generation MINI (R56) that was made in the first few years (2007 and 2008) and you are looking for parts online, it's going to get confusing. I keep seeing R52 (the first gen convertible 2004-2008) parts on Amazon and R56 buyers are buying the parts, then are confused/mad when they don't fit. That's because MINI made 2 completely different models in 2007-2008 and non-MINI people don't seem to get that you just can't call something a 2007 MINI Cooper part without asking 'hardtop or convertible'.
It is worth while doing your own research into part numbers and working with vendors who know the history of the brand.
