Why !!?? --To be independent from the Septics. I remember having an argument with a yank as to why we needed our own GNSS when the American system was perfecly good enough.
Can you imagine the fuss the yanks would make if they had to rely on a European system which we could switch off at will and would have great impact on their warfighting ability, why would they not want their own independent system, why would'nt we?
It's bad enough that we have bowed to pressure to alter the frequencies to please the yanks to allow the yanks to jam the signal without affecting their GPS signal IIRC.
As to the cost, is it better to push the money out to industry and potentially create hundreds of jobs as well as potentially safeguarding hundreds of jobs in the aerospace industry or leave it in a bank/banks to potentially be frittered away on smaller projects which could amount to much less overall. It (money) all just swirls around and around in the system anyway, it is never "lost".