Write a list of this you will always need to take and one of things you will want to chop and change. Depending on what things you will be doing/what weather you expect. Save you packing every thing and never using most of it.
Pack your always stuff in storage box or boxers. That way all you need do is load up the boxer and it's all ready to go. Thats how we do it so it's easy to change over the van when we go off grid. It will help with packing up and storage on site.
It will take a few trips before you get your set up and pack up right. It's good to decide and spit the jobs up before you arrive it saves on any "words" once you're onsite.
Pack a just in case box, I use a small tool box with water proof tape, cable ties, extra rip cord ect. Never know what you need to a running repair.
As for sites I know a few good ones in north yorkshire , we mostly use CCC Or affiliated site and temporary ones. They are basic but cheep and often in locations that are you don't find official sites.
It's worth joining CCC for the "social camping" and affiliated sites. The main sites are too big for us we like smaller sites with room to move.
Have a look at leeds DA site for the sort of "social camping" on offer. Lots of other DA sites but leeds has a good set out site.
Other site a must are ukcampsite.co.uk , lot of sites listed many with user reviews and great site search tool. The forum is friendly and helpful too.
https://www.freedomcampingclub.org/ free to join and lots of smaller out the way sites. Many are basic but nice get away sites.
You also get free night away each year once you sign up.
https://www.pitchup.com/ great one stop site for searching , finding reviews and booking all in one place.
Have you though of having a dry run / over night at a site local to you so you get a go at pitching and take down before you go proper?
If you going off grid a lot a power bank is worth a look. We on eco flow you a lot off grip and around the house and garden for portable power. They are not cheep but often have offers the real plus I they charge quicker than most others. The other plus is you can still have one or electrical "treats".
If after a time you think about getting an electric hob go for Vango sizzle induction cooking hob. They are bullet proof and built for outdoor use and are lower powered so wont trip EHU.
It's worth checking
eBay for a porta potti , lots of people buy them and never use them. We got one for a song never used or opened.
Go for the greener chemicals and one that you can use in both top , bottom save's carrying two lots. We use Fenwicks and find it work well and not over powering smell.