Well, 18-19 is actually around 65F, not 72, so that may be one reason, but the other is that US heating is usually hot blowing air from a vent and here it's radiators, so the air in the US is also dryer. The general air in the UK is damper than most places in the US, which makes it feel colder to begin with, and then you don't have the heating drying out the air to make it more comfortable. I agree with everyone else that you should switch suppliers and take your own meter readings. Tbh, £31 seems way too low to me for a 3 bed place anyway!