You have some good visual navigation (pilotage) clues around Stapleford. The M25, River Thames, London Docklands etc. You could take off and follow the M25 south eastwards until you come to the Thames, turn left and follow that out towards the North Sea. Stay on the south side of the Thames to keep clear of the Shoeburyness Ranges and out of Southend’s hair. You then have the options to drop in to Manston or Maypole, Rochester or Headcorn.
If you are uncomfortable about the controlled airspace, perhaps you should invest in a portable GPS. There are (rather expensive) aviation ones, which you might use later in your flying career, or cheaper modified car ones which will run the CAA Charts via Memory Map. This latter option would allow you to plan out a route on your PC, down load it to the GPS and then have some fall back when you are flying it by conventional navigational techniques. Other facilities that will help keep you out of trouble include the Farnborough LARS. They are quite good at redirecting people when they are heading for controlled airspace.
As mentioned above, buddying up with another PPL and taking the sectors in turn is a good option. You can go twice as far for the same cost, plus split the workload. As you are going on the ATPL route and probably will be operating in a multi-crew environment in the future, it might be useful to get used to that style of flying. As Justiciar mentions, you will need to be sure that you can perform all the functions on your own and are not relying on the other person.
Safe Flying,
Richard W.