Depending on what Uni you are looking at,
I had a D for business at AS level and C for Business A level and stayed in college for 3 years. However, this did not stop me from getting my bachelors degree in Business & Management and getting on a Masters Course in Human Resource Management.
It depends on the uni you want to go to, which will answer whether you should do a foundation year or not.
Regarding the BTEC - personally i don't think you need it, and usually people with a BTEC tend to do a foundation year before their degree.
I would suggest you seek advice on the problems you are having with applying your knowledge academically. That is the whole point of the learning process. learn from experiences and develop on them. if you are not seeking advice, you wont learn and this will hold you back especially in Uni where your undergraduate degree will mainly be compromised off evaluating and analyzing content.
Stick with your A levels, seek help on your application of knowledge, don't feel like its the end of the road because its most certainly not. just work hard and look around for universities.
obviously, higher ranked universities expect higher grades, so make sure your searching for unis that are in the same ball park.
Good Luck!