OP, you can put up with a lot if crap when you’re younger, but as you get older there us nothing more important than “location, location, location.”
Seriously, of all the possible destinations in the UK, you come up with Jaywick? 
Nooooo! You need somewhere with decent facilities and shops. Somewhere with a reasonably uplifting vibe where you don’t fear for your life every time you step out.
Do you need to be near the sea specifically?
Do bear in mind that many UK coastal areas are dead as a dodo in winter. Some people like the grey, moody effect though, so if you’re one if these, then by all means go for it.
If you’re going to be by yourself mainly when DS is at uni, then please go somewhere you can meet people and there are things to do on your doorstep. You need somewhere with a good vibe and potential for community. And somewhere safe.
Do you really need an office if you live alone? Personally I’d trade one less bedroom for a better location.
The East Coast is bloody freezing. Places like Devon and Dorset are milder, but not much. Cornwall can be lovely but miles away and also it rains every five minutes.
I wonder if Sussex could be your answeras it’s not far from London? As a single woman, I’d take a flat in Brighton or thereabouts, rather than a mansion in Jaywick any day. It doesn’t have to be Brighton, I’m sure there’s lots of reasonable places along the coast in Sussex or Hampshire and there’s fast trains into London?
I would rather live in a mobile home in Devon with sea views than that house and it would probably be better built. As long as you have pleasant views and a good location, is size that important for a single person?