I faced almost exactly this situation yesterday, except that I would have called it an IFR departure. There was also an IAP (or several) at the departure field.
I didn't go, because I was worried about being above the freezing level and in cloud. Had the freezing level been higher (as it turned out it was, later on), I'd have gone. Legally, I'm sure.
It's probably in the UK AIM, but there's no index to that, and I rarely succeed in finding what I'm looking for in it (short of reading the whole pesky thing each time).
I've flown a lot of "IMC Rating IFR" in the UK, and ask for a RIS whenever I can get one. It's extremely rare for there to be anything else around in those circumstances, so the "big sky" theory protects me.
I also avoid routing over VORs (honeypots) or at round multiples of 500 feet, or at 2400 feet round the LTMA.
Not departing IFR? The only reason would be if you were in something over the magic weight limit, at which point you'd incur hefty Eurocontrol charges. So you depart VFR and avoid the charges, then change to IFR when airborne.