Few barrels of Crude oil or LNG can give you good profit to cover all your expense..lol jus kidding..

accommodation and food are the major expense ..as you are staying with your relatives that’s covered. Need to spend only for sightseeing and tube/bus tickets. You can get discounted all day ticket. Tickets for entertainments and sightseeing is bit expensive compared to other cities. a boat trip in Thames and a bus ride in the city is all worth the money...and of course wax museum is a must see. If you are interested in nature...visit country side its so green and peaceful