Visted the channels islands in May for the Jersey Air Rally. Flew Edinburgh - Plymouth - Guernsey - Alderney - Lessay - Jersey.
All very straightforward from what I can recall. Majority of our clearances in the Jersey Zone were not above 1000'
When approaching Guernsey we were asked to route via North East Point and then right base join for 27.
Just remember to ask for SVFR clearance to enter the zone.
12 hours special branch is only if your point of UK departure / arrival is not designated. No need to notify Guernsey (other than flightplan) as far as I can recall.