Whilst London termini are generally very nice buildings, thew first few stations outside of termini might offer better photographic opportunities, the chance to see freight and also possibly meet other enthusiasts who will maybe know about 'rare' moves. In no particular order:
Stratford is good for half the stuff leaving Liverpool Street plus London Overground, Elizabeth line, DLR and two tube lines (also C2C at weekends). West end of platforms 9 and 10 are a good spot.
London Bridge - everything from Canon Street, Charing Cross plus terminating services and cross London Thameslink.
Clapham Junction - everything from Waterloo plus Southern stuff from Victoria. Also Overground and freight.
Acton Main Line, Ealing Broadway, West Ealing, Hanwell, Southall and Hayes and Harlington will get you all stuff from Paddington plus Elizabeth line and freight. West Ealing has a shuttle service to Greenford from platform 5 (only 1 diesel unit needed to work the service), Hanwell is a very pretty station in GWR colours, whilst Southall has a shed with steam locos sometimes visible.
Willesden Junction for everything from Euston plus lots of freight and Overground services. Stand on the footbridge outside the station. Not the best place for photos and not the nicest area of London. For photos suggest South Kenton which has all the Euston departures, freight and Overground.
Haringay is good for all departures from Kings Cross and Moorgate plus some freight.
Shadwell on the DLR is good for all departures from Fenchurch Street. Alternatively try Barking which also has Overground and some freight.
Rainham (Essex) has a footbridge for photos of all departures on HS1 (Eurostar and class 395 Javelins) from St Pancras.
West Hampstead Thameslink, Cricklewood or Elstree and Borehamwood get all non HS1 departures from St Pancras plus Thameslink.
There is a good amount of variety to keep the camera busy and very often something 'unusual' if you know where to look. Search 'Railway Herald railtours' for a listing of charter trains, often steam. If you know how to use realtimetrains it is also relatively easy to identify what might be unusual.