spring half term is one of their most popular times. we always used to go then and the week after. if you want a specific crossing - we always did portsmouth/st malo then this will be very busy and you wouldn't get a cabin even if you managed to book a crossing - which some of the operators struggled to do. i always remember most of the mobiles being full at may half term.
guess it depends how flexible you are on sites and crossings. But unless you are prepared to take anything i think you will struggle booking anything last minute
Do you mean Whitsun (i.e. late May/early June)? If so, I would check out booking direct with sites or using one of the Dutch operators (Selectcamp, Vacansoleil, Rent-A-Tent) rather than booking with Key/Eurocamp, even on a discount. You could save a couple of hundred quid as it's still very much low season in Europe but the UK tour operators whack on a school holiday premium. Depends how you get there too as you get a ferry/eurotunnel discount with Keycamp et al but if you want to get flights you'll need to book now to get Easyjet/Ryanair cheap deals.
Which site are you looking at and what's the Keycamp offer?
we've done keycamp twice already but this is the first time we're restricted by school holidays so don't know if we should be jumping on a discounted offer this early on? or do they do cheapies close to the time too?