I think you could make this trip much easier if, instead of making a line, you made a circle. What I mean is that after landing in London, you go straight away from it. Why not turn east instead of south?
So, after Paris, you could travel through Belgium, Germany, The Netherlands or Denmark. From any of these countries, you could quickly and easily get back to London. From Copenhagen, there is a ferry (you will get a cabin and sleep on the sea). From the Netherlands, there is also a ferry (very cheap) to England. From Belgium and Germany, there are trains to go back to London.
Sometimes it is easier to change the destination than the transport.