Is it expensive to move a combi gas boiler from out of the front bedroom and install it into the kitchen downs?

It will be fairly expensive and there will be a bit of upheaval. It begs the question why do you want to move it? Modern fan flue boilers are 100% safe in bedrooms. You should set up a maintenance contract with a Gafe firm of course. But the location of the boiler is fine, unless you must move it for a very good reason.