Ah - yes, of course - Finsbury Park! It was silly of me to forget that; Northumberland Park is irrelevant. So therefore all Underground lines - other than the W&C - can be reached from "the outside world". Though even moving the tube stock itself around might not always be easy, given different constraints on tunnel clearances, relating to carriage length etc. But that's not the point at issue in this thread. The routes for both the Northern and Victoria are necessarily via the Piccadilly [and then via either Met or District], and I guess the Jubilee is only via the Bakerloo; the Central and all the SS lines are a doddle of course.
So I make the list of LU connections (using Carto Metro):
Bakerloo Line
- Queen's Park: to Watford DC Line
Central Line
- Ruislip Depot: to Chiltern Main Line
Circle Line
- (via District Line at High Street Kensington, Gloucester Road, South Kensington, or Minories Junction)
- (via Hammersmith & City Line at Aldgate Junction)
- (via Metropolitan Line at Baker Street)
District Line
- Barking: to Tottenham & Hampstead Line
- East Putney: to Windsor Lines
- Wimbledon Park and Wimbledon: to South Western Main Line
- Gunnersbury: to North London Line
Docklands Light Railway
N/A
Hammersmith & City Line
- Barking: to Tottenham & Hampstead Line
Jubilee Line
- (via Bakerloo Line at Baker Street)
- (via Metropolitan Line at Neasden or Wembley Park)
Metropolitan Line
- Harrow-on-the-Hill: to Harrow Line
- Amersham: to Aylesbury Line
Northern Line
- (via Piccadilly Line at King's Cross Loop)
Piccadilly Line
- (via Central Line at Ruislip Depot)
- (via District Line at Acton Town or Hammersmith)
- (via Metropolitan Line at Rayners Lane)
Tramlink
N/A
Victoria Line
- (via Piccadilly Line at Finsbury Park)
Waterloo & City Line
N/A
And, for fun:
Elizabeth Line
- Pudding Mill Lane: to Great Eastern Main Line
- Plumstead Depot and Abbey Wood: to North Kent Line
- Portobello Junction: to Great Western Main Line
Overground
- South Acton Junction: to Kew Branch
- Acton Wells Junction: to Acton Curve and Dudding Hill Line
- Clapham Junction and Ludgate Junction (also to Ludgate Line): to Windsor Lines
- Latchmere Junction: to Kensington Line and Brighton Main Line
- North Pole Junction: to North Pole Depot
- Kensal Green Junction: to City Line
- Camden Road: to Primrose Hill Branch and North London Incline
- Canonbury West Junction: to Canonbury Curve
- Lea Junction and Channelsea Junctions (also to Channelsea Curve): to High Meads Loop
- Camden Junction (also to Primrose Hill Branch), Kensal Green, Willesden TMD, Mitre Bridge Junction, and Watford Junction: to West Coast Main Line
- New Cross Gate: to Sussex Main Line
- Old Kent Road Junction: to South London Line
- Junction Road Junction: to Tottenham North Curve
- Harringay Park Junction: to Harringay Curve
- South Tottenham: to Tottenham South Curve
- Woodgrange Park Junction and Romford: to Great Eastern Main Line
- Barking: to Tilbury Line
- Bethnal Green (also to Great Eastern Main Line), Hackney Downs, Clapton Junction, and Cheshunt: to West Anglia Main Line
Emirates Air Line
N/A