Hi from another Troll owner.
Using the M40 will help you avoid the Toll on the Dartford Crossing, onto the M25 at Junction 16 direction Gatwick, stay on until junction 7 where you turn off onto the M23/ A 23 direction Croydon (not Brighton), then follow the A23 you will pass Purley , then Croydon,and then use your sat nav it's only a few miles from here. A23 is the old London-Brighton road and is plenty wide enough for you, you shouldn't have any problems.It should take you on the A 212 up to Crystal Palace, look out for the old TV mast, it's right near there.Hope this is useful.
Have just come back from the Alpes Maritime , Switzerland and Italy with my Troll, if the car can get through so can you, be confident!!
Have a good trip.