Does a day trip on Eurostar from London to Paris give me enough time to see all the sights such as the Eiffel Tower, Arc de Triomphe, Les Invalides, the Tuilieri Gardens, the Louvre, the Bois de Boulogne. No, of course it doesn't. So why do you think it would work the other way round? C'est fou.

I would stay in London for a couple of days if I were you. Eurostar do excellent value for money rail/hotel packages

London is one of the great historic cities of the world.

It is also quite large.

If you propose to "see London" on a day trip then you have no idea of its magnitude.

A minimum of a week is required to see London, especially if you include the places you have mentioned.

For example you could easily spend the whole day just at the British Museum.

You can see all the highlights of London on the hop on hop off bus. Bus guide in London very nice perfect for one day. You need more like to see all the places but on this trip let's us pick places to return for longer trip.

If you can only spare one day then catch the London bus tour which takes you round all the landmarks - they are good value for money and you can hop off and hop on again without paying extra (keep your ticket somewhere safe) if you want to see more of any particular place

At least 3 or 4 days required

Certainly not. It will take you all morning just to get there. You need to plan on being in London for several days.

If you want to leave at midnight, sprint round London Usain Bolt style, then you might just get back in time!

it would take Years to see all of Historic London and that is only on the Surface

If you want to see all that in a day, I suggest you take a helicopter and look at it from the sky.