I found these reduced in my local Tesco (Banbury) this morning.
It may be a local only deal but it looks like they are reducing the old models of the TP Link adapters as they also had a wifi kit available for £17 (WPA4220) which I grabbed as well.
These are the 500Mbps models but with a firmware update they can run at 600Mbps.
There was only one on the shelf that I bought, but they had another of the wifi kits if that's of any use to anyone.
Instore deal - See link for more info only