Guys
All UK forces abide by national guidelines regarding the speed thresholds for enforcement.
These figures are published by Hertfordshire Police, but certainly South Wales and other local forces comply with the same, even if they don't openly circulate the fact.
The threshold for all enforcement is 10% + 2mph. Then up to 10% + 9mph gets you a National Speed Awareness Course. Up to 10% + 21mph means a Fixed Penalty Notice (or RIDE where available). 10% + 22mph and over earns you a trip to the magistrates (Report For Summons).
If this is confusing:
30mph limit >> 35 - 42 = NSAC, 43 - 54 = FPN, 55+ = RFS
40mph limit >> 46 - 53 = NSAC, 54 - 64 = FPN, 65+ = RFS
50mph limit >> 57 - 64 = NSAC, 65 - 74 = FPN, 75+ = RFS
60mph limit >> 68 - 75 = NSAC, 76 - 84 = FPN, 85+ = RFS
70mph limit >> 79 - 86 = NSAC, 87 - 94 = FPN, 95+ = RFS
The important things to realise is that below the NSAC figure, the offence is ignored. No action, nada. Relax. This is because there is a statutory allowance of 10% in your favour to allow for speedometer inaccuracy (although all speedometers are optimistic), so the threshold is just 2mph above this.
Therefore if you should receive a ticket for below this, write a nice letter pointing out that the threshold has not been passed. The matter should then be discontinued.
The other thing is that if an officer offers you a fixed penalty at speeds above the FPN figures, take it and pay with a smile. The alternative is worse should you start to argue.

Tip - always be nice and eat humble pie when stopped even if it hurts.
Hopefully this information will be useful and mean an end to the guys and gals getting low-figure speeding tickets.
Remember, the NSAC is only available if the identity of rider is provided within time and the speed is uncontested. Generally speaking any contest means RFS no matter what the speed so that the magistrates can sort it out.
The last thing to remember is that if the speed is totally inappropriate for the circumstances (nature of the road, weather etc) you can still get done for careless or dangerous even if the speed is quite low. This option entails a whole load of paperwork for an officer, who will therefore avoid it if possible.