We had ours done a year ago. I would say:
Definitely get a loft conversion specialist, rather than a builder. Ours was really experienced and so knew all sorts of tricks to maximise head height etc.
Have a 900mm shower fitted - I never bang my elbows in our new shower and I always do in showers elsewhere!
As pp suggested, consider making the eaves space a bit bigger and the room a bit smaller - I idly considered this, but never mentioned it. Now our eaves space is fine, but you sometimes need to climb in there to reach the stuff that's furthest from the door and it would be easier and more pleasant if it were a bit wider. Also, the room is over five metres, so the carpet-fitter tried to make us have a join (we ran the carpet the other way in the end, so it is at 90° to the carpet at the top of the stairs so avoided this).
We do not find that it is cold in winter and hot in summer. Ours is south-facing, so when it's sunny in the winter it gets quite warm in there (when the sun is low), in the summer we (counterintuitively) keep windows and curtains closed during the day. We do this in the whole house and it keeps the warm air from outside outside. The insulation and new windows keep the heat out. We only open the windows once it has cooled down outside. The sun is too high in the summer to shine in through the windows too much, and by the time it's lower it has moved round. I suppose it might be hotter in there if it were west-facing.
We don't have any veluxes in the bedroom, we find it perfectly light enough without them. We do have one over the stairs though, and this makes the first floor landing lighter and means you don't have to switch the light on to get up and down the loft stairs during the day.
Our amazing loft men said the conversion would take six weeks and it took six weeks and one day. And I absolutely love the result. It feels like we've gone from a decent-sized house (3-bed terrace) to a mansion!