If you're travelling with a dog, check the pet policy in detail rather than assuming all units accept them. Ask which specific caravans or lodges are dog-friendly, whether there's an extra charge, and whether there are dog walking areas on site. This is one of the most common causes of a frustrating check-in when it's left unchecked.
Midweek arrival is the other thing worth trying. Weekend slots get booked up first, particularly for short breaks, so a Monday or Tuesday arrival often has availability when a Friday one doesn't. If your work pattern allows it, a midweek break also tends to mean quieter on-site facilities, since most other guests are working to the same Friday-to-Friday pattern you're avoiding.
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.
Timing affects cost as much as accommodation type. Travelling outside school holidays and bank holiday weekends, even by a few days either side, tends to bring costs down considerably, simply because you're avoiding the weeks everyone else is trying to book. If your circumstances allow any flexibility on dates, this is worth more than almost any other single budgeting decision.
Coast and countryside both make for a good UK break, so the real question is what kind of days you want to have rather than which is objectively better.
See the details at
https://reviews.birdeye.com/tingdene-humberston-fitties-171810662579246.