Work with the Police here in Texas US, but have some insight into UK also.
1. 10% rule (used to) be talking about the accuracy of Radar Guns (handheld). They were not (and still really not) 100% balls on dead accurate) so a margin was allowed. Also they have to have a certificate of test/calibration regularly (although it differs per manufacturer of gun itself) and if out of test date, by even a day, it is NOT legal.
2. Gatso, oh my god! Not here thankfully but....... My Dad uses a "Road Angel" I think it is, and it has saved his but, many many a time. You can download the stationary updates free from online straight into it and it picks up the "roving" cameras also.
3. I would say "challenging" a ticket is a risk but often a good "bet". An officer has to turn up and that is often more costly to a force than the fine and hence it is dropped. Works here anyway.
Good luck and drive safe.
Nick