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
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Day Tripping
If your layover is 6 hours or more, consider making a journey into town. The center of London is just an hour away on the Tube (or 15 minutes on the express train). By the Tube, consider places with direct access to the airport (any stop along the Piccadilly line). Try Knightsbridge to shop in Harrods or South Kensington for a walk around the Natural History Museum, Science Museum or V&A, museum of decorative arts. Covent Garden, with its shops and cafes, is on the Piccadilly line, as is Leicester Square, with its famous cinemas. Windsor, with its historic castle and the famous public school Eton College, is just a 20-minute taxi ride away, while the Royal Botanic Gardens, Kew, are about the same distance away in the opposite direction.
http://www.travelchannel.com/interests/travel-tips/articles/londons-heathrow-airport-guide