Best practice is that where you have a continual load to limit it to 75-80% of rated on domestic sockets which is why UK is 13->10 and Europe is 16-13A. Thats why pretty much every home/granny charger/UMC is limited the way it is. The tough leads adapter breaks that convension so you use at your peril and as said before you really will be unpopular if you knacker somebodies electrics. I strongly advise against going that route.
Commando is not a domestic socket so you can use the full amount unless someone has fused it lower (but thats a local issue not a standard issue). Thats why a 16a commando takes 16a.
They don't just make these things up for fun.
In practice unless you're going off exploring big time each day, 12 hours at 10A is still the best part of 30kwh and in a M3 in summer thats 130 miles. You may well have even longer, you may pass a super charger, you may only go exploring alternate days in which case you'd be full. Thats why IO recommend the solution I did a few posts earlier and if you have a decent extension lead it only costs something like £12 to do.
If you think you need more then go for a bigger solution but in practice you probably don't need it.