I'm assuming you already know, but in case you don't, there's now the Pi Zero W. Less than £10, but it's a Pi Zero with wifi (802.11n) and Bluetooth (4.1) with

I'm assuming you already know, but in case you don't, there's now the Pi Zero W. Less than £10, but it's a Pi Zero with wifi (802.11n) and Bluetooth (4.1) with integrated antenna.