It is very common to do day trips from London to Windsor Castle The difference between the castle and Legoland on public transport is a 15 min bus (see below for reference and details).
Is it a London Attraction? Well that depends on how you measure London.
Technically London is the city of London (the square mile)
Greater London Area
The Greater London Area 32 boroughs plus the city. The GLA transport is controlled by Transport for London (TFL) This means all areas inside the GLA are in the Oyster zones however there are several areas in the zones outside GLA. This is important form a ticketing perspective.
Inside the M25
The M25 is an orbital road ( encompasses the greater London Area however not all town inside the M25 are not part of London an example of this is Staines-Upon-Thames, which is in the borough of Spelthrone, which is in the county of Surrey )
Precedent for things outside of GLA being called London
There are a number of airports in the united kingdom that have London in their name that are not in the Greater London Area the furthest from the centre of London is London Southend at 36 miles in the county of Essex. All of the international airports with London in their names (London City, London Gatwick, London Heathrow, London Luton, London Southend, and London Stansted) are all connected via rail to major London Train stations.
So is Legoland in Windsor a London tourist attraction?
- Inside the city of London: N
- Inside the GLA: N
- Inside the M25: N
- Closer than other places with London in their names: Y
I would say it is dubious to call Legoland London.
However that doesn't mean it is hard to get there from London. Windsor and Eton Riverside is 53 min from Waterloo on a direct train. Windsor and Eton Central is between 26 and 36 min from Paddington changing in Slough. Then a 15 min Bus ride (the stop is roughly halfway between both stations.)
The Legoland Website has a full set of directions.
Note that Windsor is not covered by TFL Travel Card zones.