But a very special type of IFR: OCAS hence no ATC guidance, correct?
IFR OCAS in the UK is pretty much just a state of mind.
The actual rules amount to the low flying rule already mentioned and the quadrantal rule (does that still exist? who can keep track?) if you're somewhere where flight levels, rather than altitudes, apply (which they quite often don't).
So for example if you're at 3,001' and scraping along the bottom of a cloud this isn't legal VFR, so you just say to yourself "hey, d'you know what, I've just decided to fly IFR" and provided you're obeying the above two rules everything is magically OK again.