You will be entering the UK through the immigration using your "Thai passport" (Thai is a visa national), therefore you need to apply for a transit visa in Thailand before your scheduled travel to the UK.
No two ways about that.
http://www.ukba.homeoffice.gov.uk/visas-immigration/transitthroughtheuk/visa-to-transit-landside/
You can transit the UK landside without a visa if:
you are not a visa national; or
you hold a valid travel document issued by the UK government; or
you are a visa national or recognised as 'stateless' and meet the 'transit without visa concession - see under 'More information' below.
If you do not meet any of these requirements (because you are a visa national or stateless, and you do not qualify for the transit without visa concession), you will need to apply for: "a visitor in transit visa, if you will be staying in the UK for less than 48 hours