Depends really, whats the closest base to you?
I've done 3 sets of work experience with the RAF, 1 week at RAF Northolt, 1 week at RAF Uxbridge (about to get details for a second week that is more officer orientated with QCS) and a day at RAF Northolt doing the officer experience. Also got some lined up with 28(AC) Squadron.
Best thing you can do is write to the CCO (Corporate Communications Officer) on the base you are looking to visit they will be able to arrange things for you and get in contact with the right people on base.(Best to find out his or her name and rank rather then just write a letter "Dear Sir or Madam" etc)
If you just want time on a squadron then its best to write to the OC of that squadron, who normally passes it on to a junior officer to arrange. Likewise if you are looking at joining or have an interest in a certain branch target your request at the correct person (ie if you want to be an Engineering Officer write you letter to OC Eng Flight on the station, likewise if you want to be an RAF Copper Oc Police Flight etc)
As for the flying...unfortunately these days its unlikely as civilians are discouraged from flying in RAF Aircraft, JHC has banned civi's from flying in RAF Heli's as of this year. Anyways you wouldnt get a shot in the backseat of an F3 or such as you need a whole days training and need to visit RAF Henlow for the thumbs up from from the Centre for Aviation Medicine there.
During my week at Northolt mainly visited different sections, 32(TR) Squadron, Met/Ops,VAS,Police Flight, AIDU, Gym,Air Traffic,. It was really a general overview.
hope this was of some help.
Will