do a shorter trip!
"What i would like to do is take my girlfriend and her parents on a trip to Le Touquet in the summer."
It sounds wonderful, but I must agree with some of the other pessimistic voices here. You shouldn't do a trip like this the first summer after you earn your PPL.
There are SO many things that can go wrong.
1) You'll be flying over the Channel on a hazy summer day. Maybe that will work out, but what if the visibility turns bad? You're not instrument-rated. It's amazing how quickly a VFR summer day can turn into instrument conditions over water. This is NOT good. I had a glimpse of this flying near the shoreline of Lake Michigan, and it was terrifying to think of flying into that.
2) You're planning to fly over the Channel in a single, piston engine airplane. The odds of surviving an engine-out incident over water are grim.
3) If you try to get back at night, your odds of survival plunge even more.
4) Your passengers probably aren't used to the bumpiness and noise of flying in a small plane. They very well may not like it! Putting them in a 172 or Piper Warrior for a long trip could be the end of that romance.
Here's a better idea: this summer take them for a shorter trip to someplace closer in the UK. There will be less time for them to get airsick, no fuel worries, no terrifying loss of visual horizon over the Channel. Just a pleasant, easy jaunt somewhere with a nice lunch and an easy flight back as dessert. If you're set on getting to Le Touquet, buy them all tickets. They'll be grateful, and in the long run so will you.