Hi Alex,
welcome to Pprune.
I fly from Elstree, too. That route will be full of students on a VFR day, so keep your eyes peeled!
The route itself is as simple as your line shows, overhead Stapleford and then direct Southend.
The RT will probably involve calling F'bo on 132.8 once you leave the Elstree ATZ for a basic service, and they'll give you a squawk. You will probably stay outside the Stapleford ATZ by staying around 2400 ft on the London QNH, although your instructor may of course want you to call them for the practice.
Southend has an RMZ, so you need to contact them around Chelmsford and tell them your intentions and you'll typically get a new squawk.
Return is the same in reverse, and you'll
be contacting the cheerful controllers - I mean AFIS - at Elstree around the Lea valley reservoirs.
But I wouldn't worry too much. If its your first XC, you're there to learn and he/she will probably do most of the RT whilst explaining what he's doing and why. The aim of the exercise is to get you used to reading the chart and recognising your local area from above - and getting used to rejoining the circuit. Try to keep a steady heading and altitude to make life easy and 99% of the job is done
But do keep a good lookout...
it'll be great fun and will start to bring all the things (weather, law, RT, performance planning etc) together - just like the real thing!
And put away that calculator and get the whizz wheel out
Have fun
B.
seriously, do keep a good lookout, it gets busy.