As said already, the exhaust in the pictures has been designed to exist only as it is shown, so polishing the ends will be the only way - or some how spray them satin black. I'd expect that most if not all genuine Milltek end trims are designed to fit only Milltek exhausts - and these Mlltek exhausts come with short tail pipes so that when the final tips are fitted, they finish at the correct point relative to the exit point in the rear bumper.
Failing that, you will be looking at fitting a Milltek etc rear box, which will cost a bit.