For those confused by British Glider registrations, a brief description may be in order.
Generally, they do not carry G-xxxx registrations although they can do, I have seen one once. Some have also had military registrations too.
The main glider register is arranged in order of the BGA number, which is a numerical sequence that refers to the gliders certificate of airworthiness, for example BGA 4947. This can usually be found somewhere on the tailplane, in relatively small letters. I prefer to organise my data on UK gliders using this as the prime reference, you'll see why in a minute.
Also issued is a three letter code, often but not always, carried on the airframe, usually in relatively large letters. For the glider in question, this code is KAX. Like the BGA number, these codes, if visible, are reliable airframe identifiers.
Not all gliders carry their three letter code and may instead carry a numerical or alpha numerical code, for example 876 or N4. These are competition numbers and although often carried on airframes, they are actually issued to pilots, not aircraft. They are also often changed so beware! They are not reliable identifiers of an airframe. The pilot may obtain a different glider and take his number with him. The number may remain on the old glider so there could be two or more gliders with the same sequence on the tail, although this is not that common.
In short, only the G-xxxx registration (typically older gliders only), military reg (for 'warbird' gliders), BGA number or the three letter code can be relied upon as a definitive identifier for an airframe. Beware of the numerical/alphanumerical codes!
For Airliners.net use, I personally use the BGA number as the registration as it is the closest thing to a typical aircraft registration (i.e. unique and logical). I use the three letter code or other visible codes for the 'Code' field. Hope this meets with approval from the screeners!
If it helps, have a look at these two examples:
On the tail of '264, just under the stabiliser, you can make out a line in the same colour as the numbers 264 - that is the BGA number, but you can't actually read it in that photo.
Other countries seem to have a less confusing system than us Brits!