Applying for a transit visa is not a simple matter, it's quite an invasive process and really not worth it unless you would also need a transit visa for Schengen, or for some reason all other flight options (probably via Schengen) cost many hundreds to thousands of £€$ more.

Plus on a return journey from the US via Schengen you most likely get to skip security.

I believe transit passengers are going to need a UK ETA in 2-3 years time while Schengen transit passengers won't need an ETIAS.