A refundable Germany-UK/Ireland roundtrip ticket with up to 6 months in the UK can get around the issue of a DL rep making an issue about no Schengen visa.
I travel mostly on one-way tickets from the US or UK to the Schengen zone, and Schengen passport control has never asked me for proof of onward travel from the Schengen Zone in order to enter the Schengen Zone. Airlines ask more often, and for the airline reps I use refundable tickets or tickets whose dates can be changed without penalty.