I agree - there is no evidence that the Mamas and the Papas ever performed in England. In fact, I don't believe Michelle ever had been to England before the fall of 1967 when they sailed there on the France after their last Ed Sullivan show appearance. According to Michelle, Denny and the various biographies and articles, the group performed the majority of their live appearances in an extensive tour during the fall of 1966 after Michelle was reinstated to the group - but, by the time 1967 rolled around they "lightened the load" as Michelle said after finding out that Cass was pregnant. Most of the "Deliver" album was recorded between January and February, 1967 and outside of a concert in Berkeley, CA in January, 1967 (as covered in the March, 1967 edition of the Saturday Evening Post) and a concert done in Hawaii in March, 1967 which was part of a vacation the group took to Oahu after "Deliver" was completed, there isn't much evidence that the Mamas and Papas played very many live appearances in 1967. Obviously this was due to Cass's pregnancy and the recording of "Deliver". But, once April rolled around, John and Michelle got involved in the planning of the Monterey Pop Festival and Cass gave birth to Owen so they weren't even rehearsing together. The only other documented live appearances after that were their closing set at Monterey on Sunday, June 18th and their final concert at the Hollywood Bowl on Friday, August 18th. They may have played a few other concerts that summer as well - although a second appearance at Carnegie Hall was not one of them as I confirmed earlier.
Rob