That in general is incorrect. Granted Briggs and Stratton make some cheap engines that are sometimes mated to cheap chassis's (sometimes plastic) for the likes of B&Q, but equally Briggs and Stratton make expensive engines that can be found on the more professional/expensive mower. I got one for my dad last year that was the best part of £500 and that as a Briggs engine on it at 190cc and its a corking engine very powerful.
You dont find these in B&Q and with a downloadable spare parts catalogue
http://www.gardencaredirect.com/product ... mower.aspx