They don't differentiate on size anymore and havent done for a few years. Especially with cars getting bigger and bigger every year.
the 3 series was their overall executive car of the year and the 5 series was the executive car cost more than 30k. So it's split up on price now.
And who needs a bigger car the a 3 series anyway?
And I could be bothered finding the "right" car but surely with several pages of 3 series with auto box under 2 years old ranging from 11k to 16k there must be a decent spec one out there which fits the OP's budget?
Better spec for you?
http://www.autotrader.co.uk/classif...radius/1501/postcode/dl120jr/page/1?logcode=p
Granted it's slightly too old though