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.
I started a thread about this yesterday. Lost lots of weight and now have absolutely no idea what I am. Anything from a 14-20. Funny when I was bigger than a 20 the sizing was much more online in all shops.
It has made it impossible to shop online and that means I have to actually go in to a shop!
I hate shopping in town on a Saturday and work all week so don't get time then.
I can be anything from an 8 to a 14. There is one make where I am actually wearing sizes 8, 10 and 12 from their ranges and they all fit! I suspect that as so much stuff is sold on the internet now that manufacturers construct clothes so they will fit a variety of sizes.