DONT wear jeans - fi they get wet they will never ever dry
tbh i dont think itll rain, it will be glorious sunshine
you could always wear jeans and take waterproof trousers (you can get fairly cheap ones from golf shops) and you just put them over the top, but you can get pretty sweaty
get a big fishing hat thing (waterproof with a big brim) instead of a brolly which would just piss you off no end
proper boots
even if its not raining you will probably be in mud
or get stylish wellies - even Asda sell them (boden if you are feeling flush)
a lightweight jacket
i bet the camping crew will be able to help
have fun!