Hornsey certainly used to be pretty full off-peak in the mid 2000s, and I'd imagine it's not that much different now. You should see a fair bit on there, but how much you'll get the numbers for is another matter.
I don't know for sure about Willesden any more either, that used to have some stuff in there during the day, viewable from Bakerloo/LO trains into Willesden Junction Low Level.
As someone else already said, Acton Main Line will get you plenty of stuff on the GWML. I'd perhaps suggest West Ealing as a possible alternative, I vaguely recall there being slightly better viewing angles there for the main lines, and you'll get the bonus of observing whatever is on the Greenford branch line.
Cricklewood will primarily be a handful of 700s and a small number of Meridians, nothing terribly exciting really. Ilford, all trains go by at a fair rate of knots so it can be difficult to ID much on there.
Grove Park CS certainly at least used to be pretty chock-a-block full during the off-peak, it might be worth investigating a spot nearby to observe ECS movements to/from it for a whole host of Southeastern trains. You can pass by the place on trains too, but getting numbers would be an entirely different challenge!
Might I also recommend spending a bit of time checking out the magnificent Paddington station too? I'm a bit biased, it being my favourite station, but it is just beautiful! I'd rate it higher than St Pancras, the building of which is stunning, but as I say I'm biased!
Favorite spots in London for me, for spotting, were:
Clapham Junction
Willesden Junction
Stratford
Bethnal Green
Finsbury Park
London Bridge
East Croydon
Ealing Broadway
I might be forgetting somewhere, but that's my list of definite good places to spot at.
Have a great trip, whatever you end up doing!
