NYMA is a budget hotel and imo not nice enough for a bday celebration. Sofitel is much nicer. I'm not usually a fan of air/hotel pkgs but if it's over Easter, that may be a good option. There are limited nonstop flight options from MAN so it makes the airfare more expensive as there's less competition. I do think a nonstop flight is worth the convenience. Right now I see no big saving for making a stop anywhere. United fares for leaving on April 24 or 25 look good to me.
Has a feature when you do a flight search on their full website on the upper left corner called fare charts that shows whether the prices are higher or lower than they have been and makes a prediction of whether they'll go up or down. Haven't checked it.
I did a fast search on kayak for air and hotel separately in dollars. The nicest least expensive 4* hotel I found was the Kitano. Putting air of $734 each and hotel together, I got $2612 for both of you which is ~£1685 for 4 nights.or £1886 for 5 nights. Just a comparison.