1) I think it's legal to take off, but I'm not certain... but only if there are sufficient gaps in the "SCT004" layer for you to get high enough to satisfy the 500' rule. It is not legal to enter the clouds.
2) If you're departing VFR, then it's not legal to do what you're suggesting, because you're not allowed to enter cloud. If you're departing IFR, then it is legal. As for sensible, that would depend how busy the airspace was, and whether it is in controlled airspace. If it's in controlled airspace (even non-radar) then ATC will provide me with at least some separation. (Assuming Class D, they will separate me from other IFR aircraft only), so I see no problem with it.
If it's uncontrolled, then you have two choices. Option 1 is what you are describing, except that you would be IFR, not VFR. I would suggest that this is only safe in very quiet airspace. Option 2 is to depart VFR, level off below the clouds, and stay there until you get a radar service. Since not many private pilots fly in this type of weather, LARS stations are generally quiet so getting a radar service, assuming you're in area that's covered, won't be a problem.
3) Can't think of any reason for departing VFR in these conditions, unless, as I described above, you're planning on levelling off before getting radar service (which you can't do under IFR in these conditions because of the 1000' rule).
I think, though, that most of these problems answer themselves once you understand the difference between VFR and IFR in UK Class G airspace. Basically, you can switch between the two at will. There is no requirement to tell anyone which set of rules you are flying under at any given time. There is no requirement to be talking to anyone if you're IFR outside of controlled airspace. The only significant differences (apart from the obvious about flying in clouds and out of sight of the surface) are the 1000' rule and the requirement to fly at quadrantal levels if above 3000' when under IFR.
FFF
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