I know several UK dealers who are repected all across the UK(and beyond) for installation work.
For a big job, I would suggest you obtain quotes from at least two - just to get a yardstick against your initial choice.
I would also insist on a breakdown of main items, cables, labour, accessories etc Don't take a "round figure" quote alone.
Of particular repute are Sheffield, King Street, Chester, Dublin and Preston - and I'm sure there are many more local to you as well.