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http://www.hac.org.uk/
The HAC are a cross between a TA unit and some sort of City Livery Company/old boys club. The Regiment is steeped in history and full of odd traditions
Fantatsic social life (if you like that sort of things). They have a variety of roles ranging from playing in a band to doing surveillance and target acquisition (STA) patrols. The STA selection is tough and involves lots of weekends in the Brecon Beacons. It has unrivaled sporting opportunities too and a rather nice TA Centre (if you can call it that!)
I met a couple of HAC chaps on a course a while back and they were decent chaps. Is often used as a stepping stone to 21 SAS and I know of at least two people who have done that.
If you want to get called up, there are plenty of chances (a HAC Tpr was killed in Afghan last year). They support a regular Artillery Battery.
http://www2.army.mod.uk/5regtra/organis ... /index.htm
As everyione else has said, there are pros and cons to every unit. The biggest advantage the TA has over all the others is that it is the biggest, therefore could offer you a broad choice of part time career. That being said, I went from the TA to the RAuxAF and couldn't be happier!
If you are in London, its worth looking at this
http://www.reserve-forces-london.org.uk ... rve06b.htm