There's a pretty simple answer to this whole thing.
- 12FPE is the absolute upper limit of power, there's no leeway for going to 12.1 FPE in law (nor does there need to be)
- The testing procedures used in determining the power of your gun aren't well defined and there's scope for a lot of variance in power output.
- Set your gun to a power level that gives you a safety margin on the upper power limit - the margin you choose is down to you.
- If your gun has AT fitted and the power is either over the 12FPE limit or over your chosen safety margin - return it to the manufacturer / retailer and ask them to either remove the AT or set the power at the level of your choosing. If you're really worried set all your guns to run at 8 FPE, you'll be plenty safe enough then.
- Sleep easy
If you can't live with that, sell your guns and give up because I can't see that it's going to change for the better any time soon.