how many amex points does airline ticket to london cost

you can transfer points to airlines miles, 1 American Express Membership reward point can be exchanged for 1 Flying Club mile. You can book a virgin atlantic ticket as soon as the miles are in your Flying Club account. A JFK to LHR award in business Class costs 80,000 miles
Most flights to london heathrow start out at 25000 points one way
You need 50,000 amex points for 1 transatlantic reward flight in World Traveller, economy class from the USA to Europe Zone 1, includes the United Kingdom, Belgium, France, Germany, Ireland, Luxembourg, Netherlands and Switzerland.
United, like some of the other airlines, runs occasional "super miles saver" promotions where you only need 15,000 miles to get a free ticket but the routes are often limited. The basic redemption is 25,000 miles for one way tickets,but often seats are very limited so you might have to use an "anytime" redemption-- 50,000 miles but you can apply it to any flight, no restrictions or blackouts.
You can also "pay with points," where you can book whatever ticket you like and pa for part/all of the fare with points. Let's say you book a 500 dollar ticket and you have 60,000 points. The 60,000 points equal 600 dollars according to the AmEx transfer rate. So, you could purchase the ticket and still have 10,000 points left over. This is a good option if you want to fly with a particular airline that does not do specific point transfers as part of the Membership Rewards program.
After selecting your travel itinerary and logging in to your American Express Card account with an eligible Card, you will be shown the number of points needed for your travel booking.