costs more per unit than C17 and is less capable than C17.
Aside from the fact that you can't buy a C-17 anymore and so that argument is moot, no it does not cost more per unit.
Luxembourg recently disclosed it paid EUR197 million (USD240 million) for its single A400M, while India has just signed a USD262 million deal for a single C-17.
As for capability, unless you're looking to airlift a main battle tank the A400M will do everything that a C-17 can do, but cheaper.
As for the UK, what does it matter anyway which is best? We have both.