Some reasons I can think of:
1. Thai is a difficult language to learn. Whereas many asylum seekers speak some English.
2. The UK's generous health and Social security system.
When you are used to a mud hut without running water, <deleted>ting in a hole in the ground, and likely having to work in the fields for long hours for a pittance, being told you are given a free house, free healthcare, free money and clothes for your children, and many of your compatriots are already there to help you, these are big incentives.
I know this isn't the case for all asylum seekers, but it is for many
Imagine the phone call back home:
I have been given a house for free. It has electricity, hot and cold running water, all paid for. They give me free money every week. I don't even have to work. The doctor, medicine and school are all free. You need to get here as fast as you can.
What puzzles me is why large numbers pass through the rest of Europe and risk their lives crossing the Channel. What is wrong with France for example? I even thought of retiring there myself.
In he end I suppose its not much different to us all living here in another country. Although most of us pay our own way