On Wikipedia's List of most expensive cities for expatriate employees (2009), note that New York is 8, London is 16 (Mercer survey)

According to Mercer's Worldwide Cost of Living survey (2010 City rankings - Top 50 cities), using New York as the Base City, Paris and London tied at 17.

Cost of Living Indexes Difference - London vs. New York and Paris (From Numbeo.com)

London vs. New York

Consumer Prices in London are 3.69% lower than in New York, NY
Consumer Prices Including Rent in London are 3.96% lower than in New York, NY
Rent Prices in London are 4.23% lower than in New York, NY
Restaurant Prices in London are 7.19% lower than in New York, NY
Groceries Prices in London are 15.91% lower than in New York, NY
Local Purchasing Power in London is 4.69% lower than in New York, NY

Restaurants

New York City

Meal, Inexpensive Restaurant 20.00 $
Meal for 2, Mid-range Restaurant 61.67 $
Combo Meal at McDonalds or Similar 6.05 $
Domestic Beer (0.5 liter draught) 5.33 $
Imported Beer (0.33 liter bottle) 7.00 $
Coke/Pepsi (0.33 liter bottle) 2.33 $
Water (0.33 liter bottle) 2.00 $

London

Meal, Inexpensive Restaurant 15.98 $
Meal for 2, Mid-range Restaurant 67.76 $
Combo Meal at McDonalds or Similar 7.65 $
Domestic Beer (0.5 liter draught) 5.13 $
Imported Beer (0.33 liter bottle) 5.09 $
Coke/Pepsi (0.33 liter bottle) 1.76 $
Water (0.33 liter bottle) 1.61 $

Markets

New York City

Milk (regular), 1 liter 1.69 $
Loaf of Fresh Bread 2.08 $
Eggs (12) 2.13 $
Fresh Cheese (1kg) 10.00 $
Chicken Breasts (Boneless, Skinless), (1kg) 9.12 $
Water (1.5 liter bottle) 1.67 $
Bottle of Wine (Mid-Range) 10.50 $
Domestic Beer (0.5 liter bottle) 2.53 $
Imported Beer (0.33 liter bottle) 3.50 $

London

Milk (regular), 1 liter 1.41 $
Loaf of Fresh Bread 1.57 $
Eggs (12) 3.32 $
Fresh Cheese (1kg) 8.79 $
Chicken Breasts (Boneless, Skinless), (1kg) 10.22 $
Water (1.5 liter bottle) 1.41 $
Bottle of Wine (Mid-Range) 10.99 $
Domestic Beer (0.5 liter bottle) 1.80 $
Imported Beer (0.33 liter bottle) 2.20 $

Transportation

New York City

One-way Ticket (local transport) 2.29 $
Monthly Pass 92.00 $
Taxi (5km within center) 13.17 $
Gasoline (1 liter) 0.84 $
Volkswagen Golf 1.4 90 KW Trendline (Or Equivalent New Car) 21,200.00 $

London

One-way Ticket (local transport) 3.91 $
Monthly Pass 144.71 $
Taxi (5km within center) 17.16 $
Gasoline (1 liter) 1.81 $
Volkswagen Golf 1.4 90 KW Trendline (Or Equivalent New Car) 28,730.56 $

Utilities (Monthly)

New York City

Basic (Electricity, Gas, Water, Garbage) 183.33 $
1 min. of Prepaid Mobile Tariff (no discounts or plans) 0.34 $
Internet (2 Mbps ADSL flat) 43.74 $

London

Basic (Electricity, Gas, Water, Garbage) 149.08 $
1 min. of Prepaid Mobile Tariff (no discounts or plans) 0.17 $
Internet (2 Mbps ADSL flat) 20.91 $

Sports And Leisure

New York City

Fitness Club, Monthly Fee for 1 Adult 73.33 $
Tennis Court Rent (1 Hour on Weekend) 60.00 $
Cinema, International Release, 1 Seat 12.83 $

London

Fitness Club, Monthly Fee for 1 Adult 97.43 $
Tennis Court Rent (1 Hour on Weekend) 19.82 $
Cinema, International Release, 1 Seat 14.15 $

Clothing And Shoes

New York

1 Pair of Levis 501 54.00 $
1 Summer Dress in a Chain Store (Zara, H&M, ...) 38.00 $
1 Pair of Nike Shoes 79.00 $
1 Pair of Men Leather Shoes 119.00 $

London

1 Pair of Levis 501 98.44 $
1 Summer Dress in a Chain Store (Zara, H&M, ...) 49.22 $
1 Pair of Nike Shoes 111.15 $
1 Pair of Men Leather Shoes 127.02 $

Rent Per Month

New York

Apartment (1 bedroom) in City Centre 2,323.29 $
Apartment (1 bedroom) Outside of Centre 1,425.00 $
Apartment (3 bedrooms) in City Centre 4,467.30 $
Apartment (3 bedrooms) Outside of Centre 2,346.67 $

London

Apartment (1 bedroom) in City Centre 2,019.00 $
Apartment (1 bedroom) Outside of Centre 1,548.48 $
Apartment (3 bedrooms) in City Centre 4,740.30 $
Apartment (3 bedrooms) Outside of Centre 2,505.50 $

Buy Apartment Price

New York

Price per Square Meter to Buy Apartment in City Centre 9,881.96 $
Price per Square Meter to Buy Apartment Outside of Centre 5,500.00 $

London

Price per Square Meter to Buy Apartment in City Centre 14,477.00 $
Price per Square Meter to Buy Apartment Outside of Centre 8,686.20 $

Salaries And Financing

New York

Median Monthly Disposable Salary (After Tax) 4,300.00 $
Mortgage Interest Rate in Percentanges (%), Yearly 5.02

London

Median Monthly Disposable Salary (After Tax) 3,273.49 $
Mortgage Interest Rate in Percentanges (%), Yearly 5.00

This cost of living data retrieved on September 21, 2010 by http://www.numbeo.com/ per the Creative Commons Attribution-Sharealike 3.0 Unported License (CC-BY-SA) and the GNU Free Documentation License (GFDL).