I have also faced the same dilemma that my current car needs its first service but have a new car just built and sitting at Emden waiting for a ship.
In my opinion there is no dilemma. Current car needs a service. £154.02 job done. Service book stamped.
Result warranty intact, no potential arguments about missing service history when trading in or selling and a nice cup of coffee,free wifi, poke about in a few nice shiny new cars in the showroom and a chat with my salesman.
Not a bad way to spend a couple of hours on a Saturday morning
