Currently the time zone options available in Personal settings > Settings include 'GMT Greenwich' and 'GMT+1 Europe/Lisbon'. I am in the UK and selected the obvious one - 'GMT Greenwich'. Indeed it is the only zone mentioning a place in the UK. It turns out that this zone does not adjust for daylight saving time so gives wrong results in the summer. Counterintuitively I should have selected 'GMT+1 Europe/Lisbon'. I suggest you change the name of this zone to 'London/Lisbon' or 'UK, Europe/Lisbon'. I suspect that most of your UK customers would, as I have done, select the wrong zone as set up at the moment.