Any NY flight was always going to be a 757 so I'm not sure why people are complaining so much - it isn't even a very long flight. The only other alternative would have been a 767 which would be far too much capacity.
£370 is an absolute bargain fare, lets hope some of the higher fare buckets also sell however as this will be key to the success of the route. Interesting to see how much APD, and other taxes, make up such a large proportion of the fare.