I think every one makes riding London to Paris too complicated. What luggage do you really need? Change of shorts, deodorant, credit card, toothbrush, sunscreen, phone & charger, simple tools and 2 spare tubes? I'd happily cycle to Paris with my Barley, or one of other
carradice saddle bags, though it turns out I had my larger saddle bag when I rode to Paris. Book 2 nights in a B&B say 40 miles out of Dieppe, cycle down to Newhaven, catch the ferry, cycle to the B&B, spend the night, in to Paris the next day, train back to B&B that night, and cycle or train back to Dieppe the next day. No need to over think it.
That bike looks like a good starting bike. The saddle may need changing, or it may not. No amount of money will guarantee you a stock saddle that suits. See how you get on. As for tyres, most bikes are sold with pretty crap tyres. You'll know it's time to change them when you keep getting punctures, but even if they are good tyres, you'll need to change them every 4,000miles or so. Tyres are consumables. Good thing is, I find Rubinos brilliant, and they are cheap. £7 each from
http://www.ribblecycles.co.uk/vittoria-rubino-3-rigid-tyre/ and you can get theme with yellow stripes, which will look great on your bike.
If you can, go to a Cycle Republic. It's
Halfords specialist brand, and they let you take bikes out for a road test, which I don't think Halfords do. Make sure the roads are dry when, no one will let you test a bike on wet roads - and call to make sure they have the bikes you want to try in stock.
I'd say you really should buy it. Falling in love is half the battle, and you are in love. It's a better bike for you than one that objectively better, but you don't love.
Edit: here's a photo I took most of the way to Paris. The big bag is because I was staying with friends in Dieppe, and need changes of shoes and clothes to be a good house guest. You can also see my leather saddle - I couldn't cycle more than 100km on the saddle that came with the £800 bike - and my red Rubino tyres.