Two simple answers : a straight line takes you through Stanstead zone, and the chances of a VFR transit through their overhead are, in my humble experience, pretty slim : or you route around. The simplest, non-transiting, non-very-low-level-corridor route will be something like East Mids-Cambridge-Earls Colne-Sarfend, and I'd talk to (assuming a week day) East Mids, Cottesmore, Cambridge, Essex and Southend.
You could go west of Stanstead, either transiting through the Luton overhead (which you'll almost certainly be approved for, as long as you're confident with your RT and can keep heading and height accurately - can you ?
), or underneath the Luton CTA (eg BKY - BPK). However, these are all increased workload routings, and whilst that's good for your experience, the very fact you're asking this question indicates you might want to try the simpler options first ?
As to your question about flight plans, you don't need one if you're going VFR outside of controlled airspace.
However, I really think it would be a good idea - rather than to take advice from here - for you to spend some time talking to your CFI about this before you try this trip.
Be safe and enjoy !
FF
