If you really want the real thing, then "King of Spirits GOLD" is the only one I know about, se
http://www.absintshop.com/products/absinth/kinggold/
The Absinthe sold over the counter in England, like Hills Absinthe is not the real ting. It do not contain more then 10 mg/l thujon. Original absinthe was 50-200 mg/l thujon. But as the fake is 65 to 75% alcohol you will get drunk, but not intoxicated on thujon.
Also se
http://en.wikipedia.org/wiki/Absinthe