In UK, power is usually delivered to camping pitches by a very heavy duty cable, with a blue tubular 3 pin plug on one end, and a blue tubular 3 pin socket on the other end. Neither end will fit your normal 13amp, 240 volt mains plug, which you presumably have on your charger. It is the responsibility of each camper to provide his/her own cable - the campsite does not provide them.
I suppose you could make up a cable with the correct blue socket on one end and a 13amp domestic socket on the other end, but it would be a faff, and to have the cable long enough to stretch from the electricity "bollard", which often serves 4 pitches, you would probably need 15 metres of heavy duty cable, which will be heavy and bulky to carry about with you.
In Northern Europe, some sites use the same types of plugs as UK sites do. Others use bollards with the local 3 pin design of sockets. In our campervan we carry a small conversion lead - 1 end with Euro 3 pin plug, the other with blue UK socket, which we can then join on to our UK standard power lead to feed power to the van.
It would be much easier to ask if the campsite staff will charge it, for a fee or a donation, from their mains power, but if they refuse then you are stuck!
One last thought - I have seen people charging phones from the shaver point in campsite washrooms. Not sure how practical it would be to attempt to recharge a whole ebike battery in that way......