FrankS,
Since you will have 4 weeks you should be able to cover England and Scotland or Wales. 3 weeks in England would be more than enough. Try not to duplicate too much. Churches and castles will become boring once you got to your third or fourth. You should try to cover more localities with special characters. Also once outside London it would be better to drive than to go by rail. It is difficult to take pictures on a MOVING train. And you won't be limited by places with train stations. With driving you can stop anywhere you like and take pictures. Countryside Bed & breakfasts are cheaper (and friendlier) than big towns hotels. If you start from London you can go up on the west side and come down on the east side and cover Scotland in the middle part of your journey. A brief directional guide would be London - Oxford and Woodstock (Blenheim) - Lancashire - Cumbria (Lake District) - Through Carlisle into Scotland - Glasgow - Stirling, Lomond, Fort William - Inverness - Edinburgh - back to England - Yorkshire (Bronte Country and York) - Derbyshire (Peak District and Chatsworth House, if you have not yet got sicked of castles by this time!) - London. If you have time left you can go south of London for day trips to say Stonehenge, Tunbridge Wells and other places in Kent. This way you will cover most of the best of England and many places mentioned in previous messages in Scotland. You can save Wales and Cornwall for your next trip. There are many interesting places there as well. But you won't have enough time this time round. I am sure your have already planned your English part. But do compare it with the above route.
You will not need your Nikonos but you do need a waterproof jacket and warm mid layer like a fleece or jumper. The British weather works in mysterious ways. It can get very hot and very cold with heavy rain and the sun coming out a few times in the same day even in the summer!
I hope you will let us know where you have been after your trip and post some pics to make us feel envy. Have a nice holiday.