I remember reading a post which said something along the lines of “the first ten nights in your new van are about learning to be comfortable”.
I think that's a very good thing to keep in mind. Renting is absolutely the right thing to do, but our first four nights in a rented Cali in inclement British weather were not the most comfortable experience we’ve had (though they were probably better than a tent would have been). The other aspects of the trip were great, and that’s what kept us going in the campervan investigations, but it’s good to be prepared.
One of the biggest things for us was finding more comfortable mattresses. For the van we eventually bought, we got two self-inflating ones, which were not too bulky and Let us control the firmness individually. Sleeping bags which could unzip fully and be used as duvets. Etc.
A thermal topper for the roof, though bulky and a bit awkward when wet, makes for much cozier evenings in the winter. Many hire companies will have those available, and I’d take one of those and any mattress toppers they can offer
Almost everywhere we’ve ever stayed has had electric hookup. We find, therefore, that a small electric kettle saves a lot of fiddling with gas, and filling a flask in the morning before we leave the site makes coffee after lunch a lot easier.
A few other accessory ideas on my blog here:
https://statusq.org/archives/2018/10/13/8664/
On your route, we liked
http://www.westfieldpaddock.co.uk/ Close to Seahouses - you can walk to the beach. Adults only. Very convenient for Lindisfarne and Bamburgh Castle. Well-maintained, but no loos or showers - we used a loo tent. It’s on the top of a rise so might be a bit windy at this time of the year!
There are one or two others on your route that we liked - we tend to go for small sites - but not open in December.
Finally, we’re members of both the big clubs, but my current favourite place for searching for campsites is ukcampsite.co.uk. Even though it looks like a website from the 90s, it has good search filtering options: we usually want showers and ‘dog-friendly’, for example, and it does list the club sites in its results.
Have fun!