The short version: book peak weeks as soon as you know your dates, and treat everything else as considerably more flexible.
Packing for a coastal caravan holiday is really about planning for a British summer rather than a guaranteed one, so flexibility matters more than any single item on the list.
More information is available at Lincolnshire coast holidays.
If you're travelling with children, pack a beach bag that stays by the door, buckets and spades, a windbreak if you have one, sun cream regardless of the forecast, and a full change of clothes kept in the car rather than the caravan, since sandy, wet children rarely want to walk back for it. Bikes are worth bringing if your park has cycle paths or is close to a coastal trail, and a car boot cool bag makes packed lunches for day trips much easier.
A UK staycation can be planned around almost any budget, provided you know where the genuine savings sit and where cutting corners actually costs you more in the long run.
If your break falls within school holidays, particularly the summer holidays or half terms, book as early as you reasonably can, ideally several months ahead. These are the weeks every family with school-age children is trying to book at the same time, so the best weeks, and often the more sought-after units within those weeks, go first. Waiting until a few weeks before a summer half term usually means choosing from whatever's left rather than what you'd actually prefer.