@GreenSkyOverMe in fact, looking at the schedules I have available (on a paid-for forum) there's 4 test trains scheduled to run today

One from Leicester via London Euston then back to Leicester, one from Mossend to Mossend via the Glasgow suburban lines (Mossend being a large freight yard just outside of Glasgow)
Rugby to Derby
Derby to Cambridge (this one is running with a 37)

@GreenSkyOverMe the flying banana is actually only one of the multiple test trains we have in the UK!

There's iirc something like 6 or 8 HST power cars used, but the actual consists they run with are not consistent, and often you'll see them run with a Class 37 and a DBSO (oldddd cab car)