I have done most of this trip over the last few years in weekly sections when I can find the time. I highly recommend it. One thing I have found interesting is seeing different areas of the country and how they are rarely as I expected them to be, such as the coal mining towns in the north east. On the map they look rather odd laid out in a grid, and I expected them to be full of unemployed hoodies dealing drugs on every corner, I was actually a bit concerned about cycling through them. When I got there, they were beautiful places with no hoodie thugs in sight at all. On the other hand, Blackpool was the hellhole I thought it would be, so sometimes I am right. Nice and flat cycling there though!
A few tips..
1. Your distance cycled will vary greatly on the terrain. Cornwall sucks monkey balls, the hills are so steep I had to walk the downs as well as the ups sometimes. 25 miles is a long way there, but I could do 100 miles easily in Linconshire.
2. Never rely on ferries, I find they are more often not running than they are. The Burnham ferry runs when it fancies, the Felixtowe ferry blew up the day before I got there and the Fleetwood ferry decided to have a maintenance day when I turned up. When this happens it can add up to 50 miles to your day. Probably not so much of a problem if you are doing the whole trip at once and you are not booking accommodation.
3. Weight is a big issue for me, not so much for others. I initially camped but found the weight made the cycling hard work. Luckily I can afford to B&B now and that makes life a lot better - Cornwall still sucked! I suggest taking the bare minimum. Don't fill your bags up with tins of beans etc. think lightweight stuff such as dry noodles or packet soup.
4. Bike. I started with a bog standard aluminium bike and it was fine. The only problems I had were a couple of spokes broke, probably because the bike wasn't designed to carry so much weight.
5. Short cuts are almost always disastrous. I took one in wales that looked OK on Google Maps but ended up with me having to carry my fully laden bike over mountainous sand dunes then up a tiny bramble covered sandy path to a mountain peak. I have also had to carry my bike over rivers where I thought there was a path, up cliffs that were not obvious on Google etc, so beware.
I hope to finish the trip next year, not sure what I'll do then, but it will have given me many great memories. I hope you will enjoy it as much as I have.