So why is there such confusion, on the part of obviously intelligent people who take the subject seriously? I think the answer lies in the difficulty in finding the answers!
I find the UK publications tortuous: I use the UKAIM and have great difficulty finding what I want in it. There's no index worthy of the name. Someone could provide a good service to pilots by compiling an index (or a whole book) that does things in logical (rather than legal) order and points to the relevant bit of the legislation for each decision a pilot needs to make. We don't all have Bookworm's fluent grasp of it all!
If I were looking to take off in marginal WX in the UK, I'd be looking at the definition of VFR in ANO Rule 26 (2)(b)(ii) - clear of cloud, in sight of surface, and in a flight vis of at least 1500 metres. I'd also be aware of the 500 foot rule (and the other mnimum height rules if they apply).
For an IMC rating, the takeoff minimum RVR is 1800 metres, so I can take off VFR or not take off at all. Therefore the issue of an IFR takeoff from an unlicensed (or non-ATC) airfield doesn't really get to me.
For the pilot with a valid IR, in the UK, it gets a bit more complex, depending on the aircraft and the airfield equipment. UKAIP AD 1.1 section 4 sets it out.
For a private flight in a single, there are no stipulated minima other than "adequate to ensure a high probability of a successful forced landing being made should a failure of the engine occur after takeoff. It should be noted that the minima for take-off by commercial air transport aeroplanes is 1000 feet cloud ceiling and 1800m RVR." So if you took off in less than those, the CAA would have a case to prosecute for hazarding an aircraft.
WCollins in his twin could, in theory, takeoff in anything down to 150m RVR if all the information and lighting equipment needed is present. (UKAIP AD 1.1 Section 4.1.2).
As to IFR departures from unlicensed fields - I don't know. I couldn't find anything about that (my copy of the UKAIM doesn't have AD 1.3 that CM referred to). Personally, I wouldn't do it (couldn't, with the IMC rating).