Force a 5 GHz connection on an iPhone 5s
iOS decides automatically whether 2.4 GHz or 5 GHz connection is best if an access point supports both.

You can change your BT Home Hub settings to broadcast two separate access points. Open the hub settings → Advanced Settings → Wireless → 5 GHz Wireless and disable “Sync with 2.4 GHz”.

            

This allows you to specifically connect to two separate access points depending on whether you wish to connect over 2.4 GHz or 5 GHz.