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Start with layers. A coastal breeze can turn a warm afternoon cool within the hour, so pack at least one lightweight waterproof, a jumper or fleece for evenings, and swimwear for the days that do turn properly warm. Even in July and August, an evening walk along the front is usually cooler than you'd expect, so don't leave the extra layer at home on the assumption you won't need it.
None of these questions take long to ask, and any decent Tingdene Holiday Parks Limited park will have straightforward answers ready. If a question gets dodged or fudged, that's often more useful information than the answer itself.
A spontaneous short break is entirely possible with a UK holiday park, you just need to know where the flexibility usually sits.
On the budget side, self-catering works in your favour with a bigger group. Cooking most meals in the caravan rather than eating out as a party of eight or ten keeps costs sensible, and splitting the shopping and cooking between the adults spreads both the effort and the spend. A shared static caravan holiday, or two units booked close together, usually works out considerably more cost-effective than the equivalent number of hotel rooms plus regular restaurant bills.
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.
Neither setting rules out the other entirely. Most coastal parks are still within striking distance of countryside walks and market towns inland, and most countryside or Broads parks are usually within a reasonable drive of a beach for the day, so you're rarely choosing one to the total exclusion of the other. It's more a question of which one you want as your daily backdrop and which you're happy to visit as an occasional day trip.