I am contemplating buying a lens that only takes rear gel filters. Is it possible to buy a circular polarising filter in gel format? If so, who can supply them (preferably UK based)?
You can get sheets of polarizing gel filters designed to be used on studio and theatrical lighting and trim a piece to fit the rear gel holder. They're not cheap, though. The main problem would be the sheets are linear polarizers and so the auto focus system on you camera would likely be disabled. The other significant issue would be that since polarizing filters are directional, you would need to cut a different piece (at a different angle) for each direction you would need, and have a way of marking each one with regards to the direction of the polarization.
Search for Rosco Polarizing #7300 Filter - 17x20" Sheet from an i.p. address in the UK and you should find someone selling them.
Although intended to create "left handed" and "right handed" light for use in filming/video recording 3D movies, Edmund Optics does offer circular polarizing sheets with a film substrate. While not gel material, the type with the film substrate can be cut to size. It may be possible to use a combination of a linear filter material such as that sold by Rosco referenced above and a layer of the Edmund material directly behind it in the rear filter holder of your lens to do what you want. Of course you would still have the constraints listed above with regard to the directionality of the linear material and you would also need to use the circular material at the proper 45º angle to the linear material. So you would need a separate set of both filters cut for the correct angle to use for each direction you wish to polarize the light!
The reason you would need both layers is because in the context of a filter used in the optical path of a camera, a "circular polarizer" is actually a linear polarizer with a quarter-wave plate behind it which transforms the polarization of the light wave into a helix shape.
For the difference between linear and circular polarizers, please see What is the difference between a linear and a circular polarizer? and the Wikipwedia article "Circular Polarization."