Extradition arrangements exist between the UK and all Commonwealth member states:
http://www.thecommonwealth.org/Internal/142227 /members/
Extradition is also possible between all EU member states:
http://europa.eu/abc/european_countries/index_ en.htm
There are (or were) bilateral treaties with all of these countries:
http://www.fco.gov.uk/servlet/Front?pagename=O penMarket/Xcelerate/ShowPage&c=Page&cid=104436 0289117
(Some treaties in that list may have been formally terminated or simply be ignored. I'm not certain, for example, that Albania would honour the 1926 treaty).
If a country isn't mentioned in those three lists, no formal extradition arrangements exist. So, for example, if you were thinking of poisoning someone with Polonium 210, it would make good sense to head for Russia shortly afterwards.
Hang on a minute, I've just noticed your user name. So, 'London Broker' is seeking a country with no extradition. I wonder why the name 'Nick Leeson' comes to mind? ;-)
Chris