The difference between AS and A-levels, is that AS is the first half of the A-level, while A-levle is the full qualification at the end of upper six (year 13). The second half is called A2. AS (first half)+A2 (second half)=A-level
You need a full A-level to go to university (minimum of 3). Some uni's might include 3 A-levels and 1 AS though.