I'm a PPL student with just 9 hours training completed. I just wanted to say that this post was really helpful to me just i terms of realising whats possible/sensible with a PPL. I constantly fantasize about what I'll get up to once(if!) I get my license and this is useful as a reality check!
Just to make clear, there are MANY things possible with a PPL, and the proposed trip is well within the grasp of a PPL holder. The issue some people have (including myself) is doing this as a first (or one of the first) flight(s) after getting ones licence.
You should have big goals and aim to achieve them, but it's good to have intermediate goals along the way to build confidence and experience. For the proposed trip something as simple as a few flights to get used to flying in the UK, some circuits to get used to a new aircraft, and maybe a trip to Northern France and back to get an idea of a foreign country and flight planning and customs/border procedures. I personally would say after doing those things that this trip is more than possible for a PPL holder in a C150 with some careful planning and making sure there are plenty of 'get out' options if something doesn't go to plan, like weather, technical issue etc - because as a newer PPL you will not always have the experience to deal with these and its better to push your limits gradually, not take a first flight off to Germany and find yourself aiming to into a small airstrip in deteriorating weather arriving with min fuel and all of a sudden finding you have less fuel than you thought and you don't have a clue of what alternates you have.
So much is possible and ambition is fantastic, it just has to be done with a sensible approach.