However, all the mechanical, and most of the digital timers had warnings
on them saying not to be used with fluorescent lights.
Why ?

Probably for the same reason my last optical mouse came with a lead
warning attached to the cord... The average intelligence level of the
world is in steady decline.

It sounds like they just don't want people buying them for non-self
starting flourescent lights because they won't work without modifying the
switch.

Took them back and bought a pack of three timers for £8.99.
A difference of £25 :-)

Peter