The OP was asking for options. He clearly thinks he's not bound by the contract he agreed to.Who on earth would sue for a £35 charge?
It's not bad luck, its failure to read the contract and to understand its implications before agreeing to it. According to the OP the T&C's states 100% charge is payable. Contrary to his assertion it is not ambiguous at all, it's very clear 100% is at risk if he breaks the contract.
If he didn't like the site's T&C's why did he agree to book it? He didn't like the holiday insurance contract becasue of its T&C'. He knows how ridgid insurers follow their t&c's, so why does he expect other businesses to be less ridgid?
Caravan sites in common with most other business have continual costs whether a pitch is occupied or not, so there isn't a zero cost impact if a camper does not attend.
Whilst I agree it's probably not worth going down the legal route for just £35, but as a principle, if you think a clause in a set of T&C's is unfair your choice is to either not enter the contract, or to seek with the other party to agree a change to the term in question, or to seek and arbitration or judgment about the clause.