Is UK Transit Visa Required For US Immigrant Visa People?

[Ariean]

My parents have US immigrant visa stamp on their passport. I booked the tickets to US with transit via UK, i am checking the visa requirements for UK transit however i am confused with the below points highlighted in UK border agency site, as the stamp on passport has this information printed as well "UPON ENDORSEMENT SERVES AS TEMPORARY I-551 EVIDENCING PERMANENT RESIDENCE FOR 1 YEAR". Can you please clarify if they are eligible to air transit via UK with out a UKtransit visa. appreciate your comments.

'Transit without visa' concession - DATV exemptions
A valid US immigrant visa packet (form 155A/155B) is a 'valid visa' for these purposes, but the following documents are not valid for the DATV exemption:
an I-512 parole letter or an I-797C (notice of action) instead of a valid US visa.
a transportation letter instead of a valid US permanent resident card issued on or after 21 April 1998;
a valid travel document with a US 'ADIT' stamp saying - 'Processed for I-551. TEMPORARY EVIDENCE OF LAWFUL ADMISSION FOR PERMANENT RESIDENCE VALID UNTIL [date]. EMPLOYMENT AUTHORIZED'; and
a US visa foil endorsed, "NOT A VISA. FOIL PREPARED AT DHS REQUEST".

[JoeF]

My parents have US immigrant visa stamp on their passport. I booked the tickets to US with transit via UK, i am checking the visa requirements for UK transit however i am confused with the below points highlighted in UK border agency site, as the stamp on passport has this information printed as well "UPON ENDORSEMENT SERVES AS TEMPORARY I-551 EVIDENCING PERMANENT RESIDENCE FOR 1 YEAR". Can you please clarify if they are eligible to air transit via UK with out a UKtransit visa. appreciate your comments.

'Transit without visa' concession - DATV exemptions

A valid US immigrant visa packet (form 155A/155B) is a 'valid visa' for these purposes

That's the immigrant visa.

a valid travel document with a US 'ADIT' stamp saying - 'Processed for I-551. TEMPORARY EVIDENCE OF LAWFUL ADMISSION FOR PERMANENT RESIDENCE VALID UNTIL [date]. EMPLOYMENT AUTHORIZED'

That's their stamp once it is endorsed.

[omshiv]

If a person has a valid unexpired US stamped visa on their passport, they need not have a UK Transit visa.

Your parents are good to go. Enjoy.