I think the point is less whether you you are likely to meet another aircraft if you fly in IMC but rather is it legal to do so. Now, I'm not that familiar with glider rules, and I haven't had a chance to check the reference that chrisN gave, but for powered aircraft it's quite simple - any aircraft can fly IFR in class G airspace (as long as it's suitably equiped and the pilot is suitably qualified). It's mandatory to do so in IMC (i.e. if the weather is anything other than VMC) and at night (there being no VFR at night in the UK).