Lessons learned so far

It is difficult to plan for a trip like this not knowing weather , road surfaces etc.
But in General (top to bottom and below)
Wear a cap under the helmet - protects again sun and rain
Water proof breathable jacket
Bring two lycra tops, wash one every evening. Don't cycle in a cotton top - too heavy and will never dry. Bring a strap to tie to it to the pannier to dry during the day if necessary.
Cotswold has a universal shower and washing soap.
Base layer on wet days (merino or mesh)
Bring a gillet
Bibshorts x 2, as with the top wash one every day. I do not find two sets of shorts at a time necessary but it depends on the quality, mine are the Endura shorts from the club. Excellent.
Comfy saddle. Mine is a Charge Spoon - excellent. Last year I used a Rolls, not as good.
Chamois creme!
Merino sox two pairs, dries very quickly and are great regardless of sun or rain
SPD shoes - you need to be able to walk in towns where the layout has been modified by the RAF. Some of them seem in need of re-designing. Where is the RAF when we need them? I also use a metatarsal bar insole which spreads the load over the forefoot.
Overshoes.
Waterproof secure panniers. Mr Baker's have gone flying a few times on bumpy roads. mine, Ortlieb , have not.
Double bar tape. Even good European roads have uneven sections and your hands get sore.
Comfortable gloves plus spare set.
Mount a bell, walkers, dogs, cyclists etc need warning as we in general are much faster than they are.
Good quality Tyres - Darren's are Schwalbe Marathon plus, mine are Gatorskin (23 mm).
At least two inner tubes and repair kit, lots of cable ties, lubricant and gloves - the chain gets very dirty!
One very light lock.
Lights fore and aft.
Very little clothing for non cycling use. This is not a fashion parade!
A selection of bars, nuts and raisins to eat en route.
I use electrolyte tablets in the water (one bottle only) - no sugar, would like to have some teeth left when I reach home.
If you use specs bring a second set (just ask Mr B!)
Sunblock (SP20 last all day) lip balm, vaseline.
Don't even consider wearing a rucksack when you cycle all day!!
Big breakfast, lunch less important , perhaps a roll or two in the bag. Big evening meal with a moderate amount of the local, in general excellent, malted refreshment juices.
Garmin AND maps - using my Garmin as a mapper leaves it out of juice by lunch time.
A list of accommodation - book ahead as early as possible!
Plenty of charger cables for camera, phone, Garmin, IPod etc ( my favourite tune is There'll always be an England belted out on my small portable speakers which I bet will soon climb up the German charts!)
Speak a few words of the appropriate language and enjoy the ride!