It's even more exciting than I thought. I've just realised that I've made a cock of my tickets.
I don't leave very late on the 13th and arrive early morning, UK time, on the 14th.
I leave very late on the 12th, ie 10 minutes after it becomes the 13th, which means after subtracting or adding 12 hours flight (I'm not sure which) and taking into account the 6 hours difference between Bangkok and the UK it means I arrive early on the 13th - a whole day earlier than I thought.
Is this time travel, or am I really stupid?