Just thought I'd add my observations on this.
I have 35% on both sides and 20% on the rear screen. According to the guy who does my tints (he did my A2 before), 75% is the legal limit.
I admit that the 20% rear at night makes it nigh on impossible to see a cyclist with no lights in town, when looking rearwards. It's not a problem on the motorway and the auto-dimming rear veiw mirror still works.
The only reason for the 20% rear is that he didn't have a sheet of anything else bigger (at the time). He offered me the choice to wait for his next delivery of 35% but I said "fit it". So it's down to me.
All I would say to any prospective "tinters", is that the next car I have will be 50%. I think this is a reasonable tint, although still illegal.
My last two cars have been 35%, but having seen my man's 50% fitted, I think this is a good compromise.
Incidentally, over the last 3 years I've never had my collar felt by the law despite a couple of occasions when I know he's looking at me and I'm expecting him to pull me up. Strange.