always book ahead. always. particularly if it is school holidays. a travel agent friend once told me "the last minute holiday deals are actually the ones no one else wants and you have to ask yourself why that is" I always do what thumbcat advises.
Changing location within the UK won't make any significant difference to the weather, the only thing that would change that would be picking a different week. Can you afford to be completely flexible about which week you go or do you have to book leave from work in advance?