Or just tick sync to GPS (which works in UTC/GMT) and set DST in summer.DST adds an hour. You'll have worked this out by now, but for those who haven't, you need to set the correct time now with DST unticked. They in March tick it to add an hour followed by unticking it in October to go back one.
Remember, Greenwich Mean Time is our winter time, British Summer Time (or as the Germans put on our dashboard DST) is GMT+1
On the 6.1 head units at least the two options being pretty much next to each other strongly gives the impression the DST tick box is for automation.