5) At one point near NewCastle (don't land there after 8:00 pm) a controller tells me to squawk 7000 and go to the next frequency. Like an idiot I comply, only to be told by the next controller to maintain VFR clear of controlled airspace. Took a few minutes to sort out the idea that I was on an active IFR flight plan on an assigned route.
Not sure exactly were you were at this point but there is a weird idiosyncrasy with the airspace over the north of England in that some of the airways actually change their MEA depending on the time of day; during the night they are lower because the base of controlled airspace drops - I have no idea why this is the case - perhaps someone from ATC could answer?
Scotland to London/the South in general is actually a flight I do quite a lot airways and when flying at c FL90/100 or whatever when one leaves controlled airspace close to Scottish/English border one contacts London Information - they will have a copy of your flight plan and will be able to co-ordinate a rejoin for you when the you meet controlled airspace further south. It can at first be very disconcerting to be told "7000, remain outside of controlled airspace" when you are not expecting it but it
doesn't mean your clearance has been lost - it just means that while you are leaving controlled airspace the airways radar units don't want to be worrying about you. At FL100 or above you can call "Military" and get a service from them I think. They may also remind you to fly the correct quadrangle level which is sometimes a +500 FL depending on you level (another silly UK only procedure that differs from everywhere else!)
As for Biggin/Thames and London I'm slightly surprised to hear about your experience; I've always found them helpful. Approach info not on the ATIS is a bit annoying but it is something one has to live with - some airports like Southampton for example put it on others don't.
In general the one thing above all else that makes flying IFR in the UK a bit of a pain is the ridgy closed and class A airways system with everything else apart from airports being G. It has the advantage of protecting IFR from the riffraff I guess but for GA ops its a pain because it often leads to one either getting stuck outside the system or chucked out of it! In the US, where most airspace is E I find flying airways in a light aircraft must easier.
As for the controllers themselves having flown in the UK, Europe and the US I think the UK's are the most professional and diligent - but not necessarily the friendliest.