I don't think there even is a 'real' size 10, etc any more.
I suppose what we refer to now is the good old days when it all made some sort of sense (t'was the 90's for me!), when there wasn't such a massive variation between stores.
As far as I'm aware, the manufacturers have 'shifted' the old average of a size 12 to a new average of 14. So today's 12's are yesteryear's 14-16's. They knew 14 was the new average size, but didn't want to alter the actual size tag in fear of putting the consumer off. Still, this hardly explains why a size 10 in Primark often fits like a size 6!
Knowing what you mean though, I would say probably Warehouse, River Island, possibly Topshop. None of which sell what I like! I just try to play it by ear, get a feel for which stores work best for me and go with whatever size that turns out to be.
Hobbs not bad, but still often hit and miss. Jaeger a bit larger than average.
Zara apparently tiny but I haven't experienced it personally, find a lot their coats & knits a bit large but skirts and pants kind of true.