As far as I know in the UK, the only way you get a CO2 emission figure for a vehicle is either through type approval (standard UK market cars) or though an SVA test (imports). [I think the value might also be an "agreed" figure for political purposes, and may not be directly related to the actual amount of CO2 emitted by that particular model of vehicle.] As Bongo/Fredas are normally imported when over 10 years old, they are exempt from the SVA test, and therefore don't get a CO2 value. Thus they can only be taxed on the original basis of engine capacity.
Younger Bongos may have gone through an SVA when imported, but whether their CO2 figures can be applied to older vehicles is debatable.
I seem to recall stumbling on a Model Report for the Bongo somewhere out on the internet - such a report usually costs a lot of money, but may be made available to help subsequent vehicles of similar type to pass the SVA test more easily. This was a few years ago now, so I'm not sure if it will have CO2 figures in it anyway ....
Some Bongo/Fredas are converted to motor caravans before first registration in the UK and are thus exempted from the SVA on this basis - but this is a bit of a minefield. Unless you've got a full conversion, you almost certainly don't fit the official definition of "motor caravan" - and if registered as one, I understand that if VOSA are doing roadside spot-checks, they can seize the vehicle there and then as it is illegal! As I said - a minefield.

Again, there's some guidance notes somewhere out on the internet to help you decide if you've got a motor caravan. Several of the photos in it are of converted Bongos, and most of them do NOT fit the requirement!
Another word of warning - I believe that all of this is supposed to have changed on 30/04/2009, and that 10 year old vehicles may now require SVA testing too. But nobody (not even VOSA) seems to know exactly what system has replaced the old one since 01/05/09 - so I think people may still be following the old rules - even though technically they no longer apply. Yup - it's a minefield!
I was very glad in the end, to buy a Bongo which was imported by someone else prior to 30/04/09 - so I didn't have to worry about all this (I don't think ....

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If you know the registration number and the Make (Mazda, Ford, etc.), you can make a Vehicle Enguiry via the DVLA web-site to find its taxable status. The CO2 figure is given - if there is one. My own Bongo shows 0g/Km. So browse eBay, see if you can find some registration numbers and check the DVLA at
http://www.taxdisc.direct.gov.uk/- it might just give you the figure you need ...... (That bloke with a notebook wandering round at the Bongo Bash wasn't me! OK?

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Don't know whether any of this actually helps with the Cyprus problem - though I believe Cyprus counts legally as part of the UK. Have you tried asking Algys Autos - they import Bongo/Fredas and have an office in Cyprus, which, as I understand it, simplifies some of the red-tape.
http://www.algysautos.com/