1) You can sit A/AS levels without sitting GCSEs or any similar qualification- there is no prerequisite to sit AS or A2 levels.
2) A-levels are split into A2s and ASs. A-levels consist of 4-6 modules, of which 2-3 are AS Level, the second 2-3 are A2 Level. The combination of all 4-6 modules gives you an A-level. Therefore, if your question is- Can I sit A2 levels without sitting AS levels? The answer is yes, you can sit all of the modules in any order. However you can cash in (certificate) either an AS, or a full A-level, but you cannot certificate the 2-3 A2 modules alone.
AS (Can be certificated alone)
A2- (Cannot be certificated alone)
A-level- Units 1-6 (Certificated to give a full A-level).