I think because the BX is so light, it's better to have narrower tyres.
When I bought my BX it had 195/55/15s on it which were awful. I got some 175/60/15 (New Mini tyres) now. I recommend these. Mine are Hankook Optimo K415s, quite, low resistance and great grip. After a year of driving they show very little signs of wear also.
This German site http://www.tyretest.com/ is great for finding a good tyre. The Germs do love to test!
Jake