Hi FHS300,
I can only answer the Cessna bit of your question:
Almost any "C100-series" plane flies by itself (Not fully loaded 205/6/7's &210's)
The classic trng ac 152/172 will never hurt you unless you force them to do so. (Assuming normal flying skills/airmanship as an FI)
I do not know the market for FI's in the UK, but can only say that having a background as an FI would be a + when shortlisting applications... (Me, that is, I can not speak for others)
Cheers, and best of luck,
Redbar1