Does a UK citizen currently violate any UK law or risk any possibility
of arrest in the United Kingdom by actively participating in the
violent overthrow of the government of a country which has no
extradition treaty in place with the UK?

Mark Thatcher is a UK citizen, as is Simon Mann

I don't think that the UK has a warrant out on either of them.

This very recent case is interesting on the subject of civil (not
criminal) liability in such as situation:

http://www.bailii.org/cgi-bin/markup.cgi?doc=/ew/cases/EWCA/Civ/2006/1370.html&query=mark+thatcher&method=phrase

Think twice 007

Extradition treaties can change; In any case, even if it would be legal,
it is not polite - unless you are a SRS (=sole remaining superpower)
or an agent thereof.

You might also consider that an extradition treaty makes it mandatory
for one country to turn over a person to another if the conditions
have been met, but any country CAN do so if it feels so inclined.  And
then there is the possibility that the latter country  - the UK in
your question -  "helps" a little, as the CIA seems to have done, and
just goes in and absconds with the person.

were you inspired by jeraboo's similar question, or are you actually
jeraboo's alter ego (actually, i guess jeraboo would be your alter
ego, since you were here first) and accidently posted from the wrong
account ("oh what a tangled web we weave...").

-cab

Barneca-ga

Yes i was indeed inspired by Jeraboo's question which you may have
noticed , i merely pasted and copied and then substituted the UK for
the US.

I have noticed from the various comments that the UK government's
attitude to foreign mercenaries differs rather remarkably from the US.
In a nutshell the UK government generally does not care a toss if its
citizens goes and fights a foreign government so long as there is no
extradition treaty.

FRDE has supplied two good examples in Simon Mann and Mark Thatcher
neither of whom have been charged with any crime in the UK. If these
two gentlemen had been US citizens , I assme they would have broken US
law by their activities.

DPRK007

BTW regarding jERABOO'S alter ego you will need to look elsewere for
an altergo ego :-)