Here is another vote for "Don't risk it" - cutting it far too fine, with no margin whatsoever for delays.

If your friend isn't free to have a birthday celebration at lunchtime then how about an early evening celebration after which you could take the last train down to Euston at 21.15 (arr 23.50)? There are several hotels around Euston - the nearest being a Travelodge which is right next door (a bit basic but perfectly adequate, I have stayed there several times)

PS Just read your post which crossed with mine... I wouldn't risk flying either... unless your transatlantic flight is on a flexible ticket AND you don't have any commitments which mean you have to be back that day.