It’s not an oddity at all. There were AC and DC versions of the Simplex 4208 panel. The K-8 school I went to for kindergarten was built in 1974 with a 24-VDC Simplex 4208 system with 4051 horns on 4050-80 light plates. (Most of the old stuff is still intact as of this typing, but there is also now a Simplex 4010 panel tied into the 4208 (for system monitoring, new duct detectors and a modular building that has Wheelock MTs.) DC systems were becoming more common during the 1970s, though I think they did cost more than 120-VAC fire alarm systems…
What IS odd how there was a Simplex fire alarm system with 4051s installed in London, England during the 1970s. I don’t know much about European fire alarm companies…