I think that bike would make a good choice if buying new. It is towards the higher end of the price scale for hybrids and you would be getting some quality Shimano parts for your money. I think it would be possible to spend less money and get just as suitable a bike, if you have another reason for liking the Scott (perhaps its looks or future use) then I'd say go for it. Otherwise, have a bit more of a dig around and compare as many manufacturers and models as you can, you may be able to save a few hundred pounds which could be spent on a helmet, some cycle-specific clothing (not necessarily lycra!), a decent lock or other accessories. Regardless, the Scott seems to be a quality bicycle from a respected manufacturer.
It seems to be set up for road and light off-road, sold as a rough tourer or "expedition" ride. The South Downs Way is very hilly, with many short sharp inclines whilst the road to Paris (from Calais or Dieppe) is relatively flat. The road surfaces are very different too, the South Downs Way being rocky with patches of loose chalk and few paved roads of any distance whilst the road to Paris is paved or tow-path style. If I were using this bike I would get a set of larger, more knobbly tyres to cope with off-road duties.
I would expect you to have little trouble completing your ride to Paris in one day, it will certainly be an effort if you're not used to distance riding. In comparison, I would expect only a very hardy cyclist on a dedicated MTB to complete a similar distance on the South Downs Way in that time.