a chronograph would give you the power output.
or look in here for a conversion
http://www.airhog.com/convert.htm

for example. your pistol fires at 430 ft/s
if you used an eley wasp pellet in .177 cal
you would get 2.957 ft/lbs, which as you can see is under the legal limit , therefor no need for a certificate.

In terms of ordering a Desert Eagle co2 pistol,
http://www.jsramsbottom.com/
they have them at £106 , only in black though