'70s book series: Submarine entombed in iceberg for 400 years, England has a druidist-like worship, and an underground "ce-tee"

Question

I'm trying to find the name of a novel or series of books that were published in the 1970s.

The book started of with a submarine crashing into an iceberg and the crew being frozen for some 400 years. When they re-animate they find London a ruin and England reverted to some form of nature worship (like druidism). A number of the crew have different adventures and one party finds an underground 'ce-tee' that still has flying helicopters (or ornithopters).

I also recall there was a thing called by a strange name (which I can't remember) but translated as 'the mystery'. Turned out to be a submarine missile launch box that only someone with knowledge of its origins could open. Think it was opened and things ended with a bang...

Answer 1

The Fires of Lan-Kern by Peter Tremayne

Lan-Kern is a Celtic world equally of both the past and the future…The crew of the nuclear submarine return from a Polar trip to find a British landscape completely devoid of life. The disoriented crew search in vain, only to find mutated animals and ruins. Then the overworked nuclear drive explodes, leaving crew member and botanist Frank Dryden alone in the forbidding world. He is befriended by the Celtic Tribe of Lan-Howlek, but soon drawn into their bloody battle with the evil followers of the witch Nelferch and he must undergo the painful and terrifying initiation rites of a Druidic healer, so that he may search for the holy relic, An Kevryn, the secret key to the Universe.