Re: Out of interest..
Gosh - what a good idea. Basically one that is already implemented in just about any GNSS chip in any phone or GPS. And probably about the level of good idea that we will get for £80 million quids worth of feasibility study.
There is no sane navigation/security issue for a BrexitPS. No feasible war where an extra 20 cms (*sorry 8 inches) left or right will make a bag of beans difference, even if we had armed forces left to project force beyond Aldershot on a weekday.
There might be a sane reason to invest public money in the UK space industry, but lets do something fun with it - like go to the Moon or Mars. We can waste just as much money and annoy the Americans and Russians to boot.