There are quite a lot of people that have done this, or are doing it (myself included). So I thought I might post a few links that Dan or others might find of use, for route planning and so on that I've not seen already mentioned.
As far as I know David Cotton was the first to document his walk around the mainland coast online (though he was not the first to walk the coast), see
http://www.britishwalks.org/walks/Named/CoastWalk/index.php He maintains to some degree a list of others who are or have done it, along with links to the website/blogs which can be seen here.
http://www.britishwalks.org/walks/Named/CoastWalk/Links.phpSome have done it all in one go, others are doing it in stages.
Ruth Livingstone is another current walker I know, she has a blog here
https://coastalwalker.co.uk/ but also a considerable list of links to other coastal walkers (see the right hand side about half way down under "Coastal Walkers") with links to their websites.
Jane Allen is another walker (who I met earlier in the year in Durness) who is doing the walk in one go for charity (currently in Cumbria) :
http://victorywalk.uk/"Helpful Mammal" is another
https://helpful-mammal.livejournal.com/tag/coastFinally, if I may, my own blog is
https://britishcoast.wordpress.com/ though I am sometime behind writing it up, I am slowly catching up. I have the west coast of Scotland still to complete, but have completed the coast of England and Wales and also the crown dependencies of Jersey and Guernsey and various other islands on the way.
Good luck Dan I hope it is going well and that you are still enjoying it all!