I think they can all detect when you are tethering, but there are ways around avoiding tether detection, probably against the rules here so google it yerselves
They all seem to have their own ways of controlling traffic too. I ditched giffgaff (o2) because to my surprise, their data network seemed slow, surprising as I always thought of o2 as being strong. Taking YouTube as example, o2 would be slow / buffering (data throttling?) whereas 3 the streams seem to be 'queued' ie: start is delayed for a second or two but after that the content plays very well throughout.
I also experienced the slowness / throttling on EE, add to that, EE would send me snotty texts threatening me about my data use. I wouldn't mind, but this was just from watching a couple of YT clips on the bus home from work
. I was previously on T-mo (EE) for well over a decade but when they started threatening me (when I was on "unlimited" monthly data add-on) I jogged them on. They may well be the new 'big thing' for 4g & that, but as a company, they suck. I would rather be with a company that specializes in 3g (3) for internet, sure they're touted as not having the best signal but for me it far exceeds my expectations.
Giffgaff seems to be the network for kids or the unemployed, lol