I think different areas of the country have different levels of acceptability regarding the % of light allowed to pass through the the tint. I'm in the Glasgow area, but I'm sure inthe Newcastle area much darker tints are permitted.
I got all of my windows tinted on my previous car, a 5 door Grand Vitara. They were to the legal limitand it looked superb. Can't remember the %. Ito had the same worry, but it was totally fine at night. I also had the matching visor stip at the top of the windscreen, which really set it off and will be going for again.
With regards to getting a tug from the police, I was never pulled over in 3 years for my tints, and mine looked darker that they were cause the interior was so dark. The police are only interested in boy racers in there blacked-out Corsas, with thebottom end of a lamp post hanging off their rearbumper.