Hey Gauvins, I first came to the UK in 2008 and fell in love with cycling here. The roads and traffic are so very different from the US (insert left-lanes & roundabouts). However, I will add that you are making a VERY big trip in a short amount of time. The UK is much smaller than the US, but travel, especially cycling, is much slower. Cycling 60 miles in a day in the US is not much, but here - it is a huge amount. Firstly, there is so much to see. Do you want to explore? Wander through a castle or abbey along the way? There is so much history on these tiny little islands - from Bronze Age, the Roman Era, Medieval, Industrial Age, and on and on. The US ain't got nothin' comparatively. Plus, there's the scenery. On my first trip, I spent two weeks cycling in Wales alone and when I came back for my second trip, I spent another two weeks cycling - mostly in Wales. There is so much to see. I would get to places such as Mumbles or Tenby or Fishguard and want to stay an extra day, just to explore - look around. And yeah... look into the Lake District (very touristy, but stunning), the Peak District, and it looks like you're skipping Corwall completely (whaaaa?) Honestly, I could go on, because I loved it so much, after my fourth trip in 2011, I moved here.
But I digress, here are some pointers. Check out
Sustrans and the
National Cycle Network. Many, many disused rail lines have been converted into wonderful cycle routes. (Many lovely canal towpaths, quite roads and snall country lanes are also incorporated...) I found the Sustrans maps rather simple, but a good general reference. What I love - still to this day, are the big ole printed maps, see:
Ordnance Survey. They will get you though anywhere, don't require batteries, a GPS signal, and are wonderful at providing the details of distance, road types, topography, and scale. Plus, I love to make notes, highlight my routes, and save the maps for later reference. (There's a wonderful feeling to be had, once you're back home and spreading out your maps on the floor... just such a cool thing to look back upon.)
For a smart phone or Garmin-type device,
OpenCycleMap.org is the single best source for route planning in the UK, IMHO. I use it along with
Ride With GPS on my Garmin everytime I go out. Also a word of warning... the hardest parts to navigate will be through urban areas. The roads can be like a squirrels nest and yeah, many roads aren't always named OR sign-posted. Asking for advice can be dicey, peeps will often point to the shortest route as car drivers - not always the safest for cyclists. Learn the difference between "A" roads and "B" roads.
Now I haven't flown in several years, but for me AND for the money at the time, I found flying to Amsterdam with a short connector to Cardiff the easiest and most interesting way to get into the UK from the US. Flying into London or Edinburgh with a bike was horrible! Personally, I found that taking the slowest fight, with the most connections (2-3) was not only the easiest, cheapest, but also the most relaxed. (I once had an 8-hour layover in Amsterdam which allowed me to visit the city for a short while - I rented a bike, rode into town, and it was fabulous!)
I too set out to travel average 60 miles per day when I first came to the UK, but soon realised that 35 to 40 was more more realistic. This allowed me to relax and not only see the sights, but to also find good places to wild-camp (which you will not find near most urban areas...). As noted above too, it's ony legal in Scotland - but often overlooked elsewhere in the UK, as long as you're a decent human being, don't set the place on fire, and leave no trace. I always did a mix of wild-camping, public/private campgrounds, and small B&B's. You will be shocked at the high standard most campgrounds have in the UK. (Seeing stand-alone public toilets still shocks me to this day.)
I live in Wales now and cycle 4k-5k a year. I have a blog you can check out;
Bike Wales - (if you're interested) and I will be glad to answer any questions you may have about planning a trip tot he UK. I hope I've helped you some already. Good luck - you'll have a great time wherever you go... because you'll be cycling. Cheers!