with regards speed signs and cameras, those average speed things are a nightmare (the M1 near me is riddled with sections of them). tbh ive seen so many drivers going over them by a lot that what you describe was fine im sure. its very hard to do an average speed for that distance and most drivers stray over a bit.
also, a speeding ticket - not the end of the world! i have one

im not proud of it but i wasnt way over the limit and got caught. fair play, i now have 3 points on my licence for another year i think before i can apply to get it removed. you can get i think its 12 points before youre banned, so its not like one incident is going to mean you lose your car (unless youre doing 150 in a 30 zone of course!)
the key thing is you sound like a sensible driver, most people arent as careful/cautious as you. just try and notice the ones you can and try to make a mental note of where the speeds change on your most common routes. it will become second nature.