According to http://www.londonbusroutes.net/changes.htm , Bus 283 has a
diferent timetable depending on whether BST or GMT is effective. Does
anyone
know why this seems to be the only route which does this?

My first thought was that it must serve an airport, since flights to parts
of the world where summer time is not observed tend to operate at the same
GMT time all year round... but no, the 283 runs around East Acton, and
doesn't go anywhere near an airport.

I see it serves the Wetland Centre which has different opening hours in
Summer and Winter and perhaps differing demands. Could it be something to
do with that?