It's not that French guys wanted to build a spaceport in the middle of nowhere in Guiana, just it's easier to reach geostationary orbit from the equator. Also if things go wrong you really need a large uninhabited place to the east.
There's a map on p. 23 of the proposed launch sites. Conveniently, there's one in each country within the UK except Northern Ireland.
For the goals put forth in the document (higher inclination orbits), I think it's not much of an impediment to launch that far from the equator. Plus I think even if you're going to GEO the penalty for launching further north isn't that bad in terms of delta-v. It's probably less than what Israel has to deal with (they only launch into retrograde orbits)
It's not that French guys wanted to build a spaceport in the middle of nowhere in Guiana, just it's easier to reach geostationary orbit from the equator. Also if things go wrong you really need a large uninhabited place to the east.