Looks like isdst should be 1 on

    Europe/London  Sat Oct 26 23:00:00 1968

and

    Europe/London  Sun Oct 31 01:59:59 1971

zdump -v -c 1968,1973 Europe/London
Europe/London  -9223372036854775808 = NULL
Europe/London  -9223372036854689408 = NULL
Europe/London  Sun Feb 18 01:59:59 1968 UT = Sun Feb 18 01:59:59 1968 GMT isdst=0 gmtoff=0
Europe/London  Sun Feb 18 02:00:00 1968 UT = Sun Feb 18 03:00:00 1968 BST isdst=1 gmtoff=3600
Europe/London  Sat Oct 26 22:59:59 1968 UT = Sat Oct 26 23:59:59 1968 BST isdst=1 gmtoff=3600
Europe/London  Sat Oct 26 23:00:00 1968 UT = Sun Oct 27 00:00:00 1968 BST isdst=0 gmtoff=3600
Europe/London  Sun Oct 31 01:59:59 1971 UT = Sun Oct 31 02:59:59 1971 BST isdst=0 gmtoff=3600
Europe/London  Sun Oct 31 02:00:00 1971 UT = Sun Oct 31 02:00:00 1971 GMT isdst=0 gmtoff=0
Europe/London  Sun Mar 19 01:59:59 1972 UT = Sun Mar 19 01:59:59 1972 GMT isdst=0 gmtoff=0
Europe/London  Sun Mar 19 02:00:00 1972 UT = Sun Mar 19 03:00:00 1972 BST isdst=1 gmtoff=3600
Europe/London  Sun Oct 29 01:59:59 1972 UT = Sun Oct 29 02:59:59 1972 BST isdst=1 gmtoff=3600
Europe/London  Sun Oct 29 02:00:00 1972 UT = Sun Oct 29 02:00:00 1972 GMT isdst=0 gmtoff=0
Europe/London  9223372036854689407 = NULL
Europe/London  9223372036854775807 = NULL