I just realized something weird in the GLX5600 and the Gulfman G9100 that I'll try to explain.
In this module, the HT is set by UTC differential, while the WT is set by city names.
I would expect that changing the DST in the HT would have no further influence.
However, I can see that changing the DST in the HT modifies the UTC !
Possible explanation : even though the UTC is configured manually, it seems that the watch updates it automatically : it calculates it as the difference between the actual HT and the actual London time in the WT. (whether or not these actual times are with DST).
So it seems that as far as the HT is concerned, changing the DST always modifies the UTC. However, I think that the UTC should be configured only manually and shouldn't be modified by the watch automatically in any case.