Correct.

For sure, you do need a visitor in transit visa.

https://www.gov.uk/check-uk-visa/y/thailand/transit/yes
You’ll need a visa to pass through the UK in transit

You should apply for a Visitor in Transit visa if you arrive on a flight and will pass through immigration control before you leave the UK.

Transiting without a visa

You might be eligible for the ‘transit without visa concession’ if:

    you arrive and depart by air
    have a confirmed onward flight which leaves within 24 hours
    have the correct documents for your destination (eg a visa for that country)

The ‘transit without visa concession’ is decided by the immigration officer at the border. You won’t be allowed to transit if they decide you need a visa, so you might want to apply for a transit visa before you travel.