On our 12-night Med cruise from Barcelona to Venice in 2014, I was supposed to meet my wife and daughter in London at Heathrow and fly with them to Barcelona the day before our cruise. (Since I had to work, they went over to London and Paris before the cruise and were flying from Paris to Barcelona via Heathrow on British Airways, and I was on the British Airways Phoenix non-stop to Heathrow (LHR) and would join them on the same flight to Barcelona. The BA PHX-LHR flight the day before mine was cancelled due to maintenance issues, so when the LHR-PHX flight arrived that was supposed to be mine, BA used that plane for the day-before-flight passengers and had our flight wait for their plane to be repaired [not sure why BA decided to make two 747s of passengers upset instead of just making the original planeload a little madder that they were delayed longer] which was supposed to be a two-hour delay for us. Needless to say, we did not get out until 6 hours later, so by the time I got to Heathrow, all of the flights that day to Barcelona had left [including the one with my wife and daughter], so BA put me up for the night and on the first flight of the morning to Barcelona, which was maintenance delayed.) Since I was supposed to meet them and it did not work out that way, they had all of the Euros and I had none. At baggage claim in Barcelona, I went over to the
DCL reps for the ground transfer and asked if I could get on it, which they did, and even better, they let me charge the transfer to my cabin.
So the point of my long story, is that you can get on a DCL transfer in Europe without a reservation on the day of embarkation; however, I would get a reservation ahead of time if that is the way you know you want to go.
For our inaugural 12-night British Isles cruise in 2016, we arrived the day before, stayed at the Heathrow Terminal 5 Sofitel, and had reservations for a DCL transfer to Dover. We met the DCL reps in the Terminal 5 arrival area just outside of the Customs area (outside of security area) for our almost 2-hour trip (including a mid-trip rest area stop) to the Port of Dover.