Even though blue plugs are marked as L and N, Europeans don't seem to take much notice of it. Maybe because most of their kit is double pole switched (as against single pole in the UK) so it matters not which way it is wired. Applies also to 'standard' Schuko two/three pin Euro outlets.
Get a blue plug and socket with a metre of three core cable and wire them through, but reverse the live and neutral at the socket end. Then if your mains tester in the 'van shows the mains is reversed all you need do is put the 'adapter' cable in line and all will be well.
Good idea also to get a spare socket and another piece of cable. When you go over the water go to a DIY shed and buy a right-angled Schuko plug (you will have difficulty getting them over here) and put it on the end of the cable. Then you will have all you need for connection almost anywhere in Europe.
[Right angled as most outlets on sites are in boxes with doors. If you get a plug in the UK it will probably be straight and rubber which will stick out and stop the door closing properly. A right-angled plug with 1m of cable on it will allow the door to close and you can connect to your blue cable outside the box.]