Sponsor letter for stop over in London ?

[Cambridge]

My parents plan to stop over in London for 1 week enroute to the USA. In London they will be staying with my cousin who is a UK resident. When applying for a short term tourist visa do they need to get an invitation or sponsor letter from my cousin ?

Thanks for the help.

[peace999]

sorry this is a US visa forum. Please google UK visa India and go from there.