Nr Fairey
Heli Routes through London are pretty well explained/shown on the Helicopter Routes in the London Control Zone Chart.ie Min/Max heights. single/twin routes etc.And which routes may/may not be available due to Heathrow etc.
However the Pages (599 thru 612 I think)in Pooleys are almost a must if you haven't been through before these explain entry procedures into the London CTR and limitations on weather/vis etc.
They are usually helpful but expect you to be very accurate in your flying and when you have looked at the chart you'll realise why!
It also gets busy, As you are doing it with the instructor who has prior experience it shouldn't be a problem,but I wouldn't attempt it on my own for the first time(Being myself only a few hundred hour novice when it comes to the art of rotary wing flying)You'll be surprised how difficult it is to follow the chart on certain routes and the landmarks to fly to/from are also important to find to give you your route.
Im sure more experinced heli routers would have more to add, these are just my experiences.
Regards
hoverbover
PS
This is my first posting hope it works OK.