Is Central London north or south of the Thames?
Google is your friend:
http://maps.google.co.uk/maps?f=q&hl=en&geocode=&q=london&ie=UTF8&ll=51.50949,-0.12557&spn=0.027939,0.080252&z=14
Seriously, though - a lot of the questions you're asking are very basic (what's a quid (a pound), how's the scenery on the Eurostar (dark), how much does an average Paris brothel cost (I really hope you meant hostel)), and a quick Google search will give you the answer.
As for how the same room can cost double when the occupancy is doubled, it's simple - if renters will pay it - and they will - landlords will charge it. Most B&Bs operate on this scheme as well.