My friend, I have a big present for you... in the form of Norwegian camping laws. It's legal to camp anywhere in the entire country, as long as it is not less than 150m from a building (without permission - Norwegians are the friendliest people I have met, they will almost invariably grant if you ask - but then you'll not need to). Similarly, it's legal to walk anywhere in the country. Literally anywhere, unless it's Royal land or Military land (mostly just the area of a barracks, not anything big like an equivalent to Dartmoor).
This to me is fantastic, one of the best laws I have known - one I make use of every time I am there. As I said in another thread, why pay for a hotel or hostel when you can wild camp anywhere you like?
Fires are permitted in National Parks only between September/October to May. Outside of them I am afraid I do not know the law. Locals will be happy to fill you in on local regs - it's always worthwhile asking.
As regards climate, it is relatively similar to the UK's in September, but bear in mind it's a sight further north than you in Kent! It can vary, I have experienced between 25 and 0 C, but packing for a UK autumn you can't go wrong. The big downside is that you're going to Bergen (absolutely lovely city, just try to avoid the places that sell viking gifts as they're stuffed with British and American tourists) - not in terms of the city itself or the area, but that it's the rainiest place in Europe. I can tell you that's very true!
Shout if you want more :biggthump
Cheers,
Chris