As a further update, there are some wireless routers which support up to 600 Mbps Wireless-N at 2.4 GHz using 40 MHz channel width but with only a 3x3 antenna configuration. One example is the Netgear AC1900 D7000 router.
These use 256-QAM modulation which provides a 33% increase in data rates compared to the more common 64-QAM. This means that when using 40 MHz channel at 2.4 GHz with 802.11n, a max data rate of 600 Mbps can be achieved using either a 4x4 antenna with 64-QAM or using a 3x3 antenna with 256-QAM. The BT Smart Hub test report states that the 290 Mbps data rate achieved on the Smart Hub at 2.4 GHz with a 3x3 antenna uses 256-QAM (but only a 20 MHz channel, otherwise it would go up to 600 Mbps) and was tested with a Macbook Pro laptop with 3x3 antenna, but to get the benefit the Macbook must also support 256-QAM.
256-QAM is used as standard at 5 GHz with an 802.11ac adapter, hence the speed increase of 11ac over 11n, but there are very few client systems which support 256-QAM at 2.4 GHz because this is not compliant with the 802.11n spec and is regarded as a proprietary extension (for now at least). It is a bit misleading of BT not to emphasise this in their test report.