another station you might have problems is London Waterloo, went there one day unannounced and got turned away, need to get Sout West Trains know in advance becuase the drivers don't like their photos taken, so got an letter sent off and on the next trip, shown the letter at the barriers staff and they let us through, we kept the letter and show it each time when i'm at Waterloo, the letter dated from May or june
Had a good laugh at this one, if you don't tell them in advance you can't take pictures because drivers don't like it but if you write in advance you can take pictures of the same trains with the same drivers in charge of them - what a load of rubbish. Should have just told them that you were taking pictures of trains not drivers. Its things like this that help explain why many people don't bother with this signing-in malarkey.
Anyway back to spotting in London, if I wanted SWT trains I would go to Vauxhall or Clapham Jct (also the latter for Southern trains.
East Coast would be Finsbury Park where you would also get the 313's that go to Moorgate.
South Eastern would be London Bridge and you would also get some Southern units there.
FGW would be Acton Main Line where you would also get freights.
Virgin and London Midland you might try Willesden although you would be best going out of the station and onto the footbridge over the lines, you would also get freight here.
East Anglia, if you just want units then it is Bethnal Green but if you want freight as well then it might be better at Stratford.
Just my opinions but might also depend on weather and whether you just collect units or go for individual coach numbers.