I kinda cycled round Britain and Ireland in 1995: 6000 miles in 4 months.
Set out to follow the coast but got driven inland by traffic. This was pre-Sustrans and so just plotted a route on quiet roads.Loved every minute, even the crap bits. Was the best thing I’ve ever done.
Most challenging bit was working out when to go. I set off in early April and it was bloody cold. By August it was hot and I was really struggling to find somewhere to camp, so headed home and filled in Norfolk a month later.
http://www.crispinbennett.freedomnames.co.uk/Round%20Britain.htm
Peter Mann really did cycle the coast. His book is well written and a fun read: Postcards from the Edge of Britain: A 5, 000 Mile Journey around the Coast
https://www.ebay.co.uk/p/88613334?iid=312770912530