I filmed the N London diversion last year (from the 'back' of the train as we left Euston) and we followed the route described by
@adrock1976 above...
Up to around the 1 minute mark is from Euston to Wembley, after that we have reversed, passing Willesden J low level around 4.04, Camden Rd around 4.50, and around 6.00 approaching the junction with the ECML.
I also filmed from a diverted service in summer 2017, on this occasion from the 'front' of the train as it left Euston. At about 2.06 you can see we have left the loco that brought us from Euston, in Wembley Yard. From about 10.44 is the return journey arriving into London. Around 12.00 we approach the old site of Primrose Hill station (just before converging with the lines coming out of Euston) so evidently on that occasion the route was the one described by
@TimboM above:
Quite a fun route do do, rather tortuous and somewhat confusing to anyone who is paying attention to where they are, but unaware of the diversion arrangements, because an hour plus after you leave Euston, the Euston tower appears at quite close hand on your right hand side!