When tethering, is there a way to turn off the phone's own data usage? Just a quick question, really.

When I'm tethering (via USB, or wireless, or whatever) my phone's apps (and there are a significant number of my apps which do this) are constantly using the phone's mobile data connection to push notifications.

My phone's mobile connection is poor at the best of times (Samsung Galaxy S3 on o2 in the North East of England), such as now, where it's constantly flicking between G, E, H, H+ and 3G.

As such, I'd like to effectively "disable" my phone for every purpose other than providing an internet connection my laptop via USB or wireless.

Is that possible?
Try Android Firewall from the Play Store. You could create a "White list" that starts off with denying all network activity for all apps, to which you'd then allow just your tethering to go through.

Then, when you want everything to go through again, just disable the firewall...Or better yet create another white list that you can switch to.