New York Habitat has properties located throughout central London and the surrounding areas. While prices may vary enormously according to the neighbourhood, and the age and architecture of the building, here is an idea of what you can expect to pay:

LONDON FURNISHED APARTMENT RENTALS (1 month to 6 months)
  Price Ranges
Studio £ 1200 to £ 2500  
One Bedroom £ 1500 to £ 3500  
Two Bedrooms £ 2000 to £ 5000  
Luxury £ 3000 to £ 10000  

LONDON FURNISHED APARTMENT RENTALS (6 months and up)
  Price Ranges
Studio £ 700 to £ 1500  
One Bedroom £ 800 to £ 2000  
Two Bedrooms £ 1200 to £ 4000  
Luxury £ 2500 to £ 8000  
LONDON VACATION RENTALS (from 1 night to 1 month)
 
Price Ranges
Studio
£ 55 to £ 150  
One Bedroom
£ 80 to £ 250  
Two Bedrooms
£ 120 to £ 350  
Luxury
£ 200 to £ 1000