Agfa "official" pre-paid processing was dealt with by the same lab at Coventry who carry out the official UK Fujichrome/Jessops/Boots prepaid slide services. (They do a first-class job
)
The Agfa liquidators arranged for prepaid envelopes to be honoured only up to 31st December 2007 (that's official, I checked with them
).
As CT100 is an E6 film, the Fujichrome lab will still process this (and any make of E6 film) at their normal prices, or any E6 lab will oblige.
Google Fujichrome UK lab, or similar search.