I get excellent mileage out of tires, at least the Pilot Road series, on a C-14. I got just over 16K out of three sets of PR2's, and a similar although maybe a little less, out of a set of PR3's. Not nearly so many miles out of Pirelli Angels (tried both ST and GT) although when brand new, I LOVE Angel STs on this bike. I got 9,3XX miles out of the original Brickstones. And while I use the whole tire, down to about the wear bars, I never, ever get past the rubber; I have never had a streak of tire carcass or belt show on any vehicle since I was a teenager and did not know better / care much / sometimes did not have the dough for tires when I really did need them.
Now I generally ride like Mary Jane Tinklepants and always accelerate, decelerate and brake smoothly. Not a fan of slapping the throttle / gear shifts / brakes ON / OFF and so even when riding fairly hard (Boys!) I try to be smooth. And a lot of my miles are on the highway (motorway) rather than in the twisties; I am the other guy on the planet who does not hate riding a motorcycle on the slab and actually enjoy it.
After having a lot of discussions with others around the country, and the world I guess, and thinking about it for a bit, I think tire wear depends on a few things, some controllable, some not. I live in the northeast US, and do not ride at really high outside temps. if I can help it (80 or maybe 85F is tops for me unless going to an event). And as I ride all year, a lot of miles add up with the temps. under, say, 60F, and a sizeable number below freezing as well. I think this makes a tremendous difference in tire life compared with those riding in the southern US, where temps. swing between hot and extremely hot. Higher temps. are really hard on tires I think. The other thing is that the paving in the south seems to be done with different asphalt than the north and west of the US. Tennessee is notorious for tearing up tires in very few miles and their road material is usually blamed for this.
I have no idea how things are in the UK other than I do not believe it gets extremely hot or extremely cold there. Otherwise, I have no idea of normal riding types nor any idea of what your roads are made out of. ?? But here in the US, there are people who cannot get 3K miles out of a set of tires and others who can get close to 20K. It does not have to make sense, it just "is".
Brian
12,000 km is around 7500 miles and that sounds/feels about right for a rear on a large sports tourer.
On the other hand 18,000 miles (or just shy of 29,000 km) sounds (no offence intended) like a pile of <insert epithet of choice>