Sixth form colleges might not like the sound of an 18 year old sitting A-Levels alongside 16 year olds, they operate more like schools. Firstly consider what it is you want to do after you finish college, there maybe a vocational option at Level 3 such as a BTEC or City and Guilds 2-year course to get you there instead.
A-Levels are not recognised by employers only universities, Level 3 vocational qualifications are recognised by both.