It because of this in the timezone database:

# Zone    NAME        STDOFF    RULES    FORMAT    [UNTIL]
Zone    Europe/London    -0:01:15 -    LMT    1847 Dec  1
             0:00    GB-Eire    %s    1968 Oct 27
             1:00    -    BST    1971 Oct 31  2:00u
             0:00    GB-Eire    %s    1996
             0:00    EU    GMT/BST
[download]

So GB (Europe/London) was 01:00 offset from 1968 Oct 27 through to 1971 Oct 31 2AM.

Apparently this was 'a three-year experiment was conducted with British Standard Time, keeping the clocks fixed throughout the year on GMT+1.'