Seems a rather long flight for such a short distance.
The Dr's letter is always handy but doubt you'll need one, however it's always best to get have one in case you travel again further afield. Some gp surgery's charge for letters, if this is the case for you then ask your diabetes consultant or dsn for a letter, they'll likely not charge you.
As for anything else, just make sure you carry plenty of glucose with you and carry sufficient supplies to see you through your trip, if your staying in a hotel then the rooms can get quite warm sometimes, therefore use Frio Wallets if necessary. Above all else, enjoy your trip and do stay safe.