Look at Wikipedia about British Summer Time experiment. Citing:
"A further inquiry during 1966–67 led the government of Harold Wilson to introduce the British Standard Time experiment, with Britain remaining on GMT+1 throughout the year. This took place between 27 October 1968 and 31 October 1971, when there was a reversion to the previous arrangement."
Remember that the strange expression format(0)
effectively relates to date 1970-01-01.
Update: Remark about OPs edits and (deleted) comments: It is a big difference if the time zone parameter implicitly given to SimpleDateFormat
(implicitly because of default system configuration) is specified as "Europe/London" (then GMT+1 in year 1970 according to time zone history) or as fixed offset like "GMT" (then of course no additional hour shift). By the way, the cited british website about greenwich mean time does unfortunately not enlighten the user about different historic tz data.