For the e11/e10/e7 loft and side return chaps- i used agl construction for my L shaped loft in 2017 and I'm very happy with it. I paid under 40k for it back then and whilst he's put his prices up since I think you'd be looking at 45k all in to a finished standard.
The bloke who runs it is a decent bloke and they also do extensions and side returns. You can also spec it as much as you want, for example if you want to source the roof windows then you can do so and that will save you a few hundred. Same with tiling. Or they can do turn key.
They're also about to do my brother in laws loft in leytonstone in March.