You are dumb at times. He is doing the insulation anyway. he just needs to put thicker stuff in. Which is not that much more expensive. TRVs? Something el;se to go wrong. Life span is 5 to 8 years on average. The fuel they save can be outweighed by the replacement cost. In many case they do improve comfort conditions.
It is not case of either or. It is a case of thicker insulation in a ceiling that is air tight.
What a wandering mind. He is putting in insulation. It is a matter of putting thicker stuff in.
If oil prices rise, which they will, then spending on insulation is very wise.
In the book EcoHouse - A Design Guide, they gave a study that calculated in
1987 200mm was the optimum in walls at the then current fuel prices.As fuel prices rise and the insulation manufacturing costs too, the whole payback calcs were scewed somewhat. So they calculated insulation levels on energy to make insulation. They came to the thickness of 650mm. The conclusion was that what constrains you is the detailing of the structure to hold as much insulation as possible.
The book says (written by three people) put in as much insulation as possible in the structure you have, or design a structure to suit.
That means if you can get 300-400mm in the loft, which is the easiest part of the house to add insulation then do it.