Unfair terms are simply void
Directive 93/13/EEC on unfair terms in consumer contracts applies to any term not individually negotiated, which is what a booking policy is. A term causing a significant imbalance to the detriment of the consumer, contrary to good faith, is not binding on them.
The Annex specifically names a term requiring a consumer who fails to fulfil an obligation to pay a disproportionately high sum in compensation. A no-show fee bearing some relation to your actual loss is defensible. One set at a punitive level to deter behaviour is the example in the text.
Note what "not binding" means: you do not get a reduced fee, you get no fee. A court striking the term leaves you with nothing.
Telling them before, not after
Directive 2011/83/EU on consumer rights requires the main characteristics, the total price and the arrangements for payment to be given before the consumer is bound, in plain and intelligible language. For a distance booking, Article 8(2) also requires the consumer to explicitly acknowledge that the order implies an obligation to pay.
In practice: the policy belongs on the booking form where the guest confirms, not in terms linked from a footer and not in the confirmation email. A policy the guest first learns about when the charge appears is the weakest possible position.
Deposit, prepayment or card guarantee
These are different things and it is worth being clear which you are running. A deposit is money taken now and set against the bill. A prepayment is the meal paid for in advance. A card guarantee takes no money unless the guest fails to show.
A card guarantee is generally the easiest to defend, because the guest pays nothing when they behave as agreed. Whichever you use, say plainly what happens on a late cancellation, on a reduced party size, and on a genuine emergency. A policy with a stated exception process reads as reasonable; one with none reads as the disproportionate term in the Annex.
Refunds are a separate hazard. If you take money and cannot deliver, you owe it back, and how quickly you return it is the thing guests complain to authorities about.