What Vapour Barrier/Vapour Control Layer/Breathable Membrane?

You do need to prevent moisture from the bathroom (and the whole first floor) making its way above, which will cause condensation. The normal way to do this is to use vapour panel plasterboard, which is basically plasterboard with foil backing. It needs to be neatly installed with no damage to the foil. When skimmed over, you will have an airtight layer. Any penetrations (lights, extractor etc) must have tight-fitting & airtight connections. The problems that you've read about with impermeable vapour barriers, normally occur interstitially in timber walls with multiple layers, it's not really a concern here. Your main concern is preventing moist air from escaping into the loft and condensing on cold surfaces. However if you do want to use a breathable or 'intelligent' membrane, you'll need to spend a lot of money at a specialist supplier such as Mike Wye, Ecomerchant, Ecological Building systems etc, and pay for pallet delivery. You'll need the (expensive) correct jointing tapes. Expect to pay about £400 and have a lot of materials left over. It doesn't sound like it's justified in this case. If you were renovating the whole house then there could well be a case for intelligent membranes throughout, but this needs to be programmed in, budgeted for and well-planned and executed.

I don't think it is necessary, unless you're installing a steam room. Yes, you can use the vapour barrier plasterboard, but in most cases moisture resistant board should suffice. All I would say is, better to invest in a good bathroom extractor that is powerful and has humidity setting as well as timer one (both adjustable). Most of the eves are built with ventilation in mind, so there is air flow (in the loft), just make sure the insulation doesn't "block" the eves (where wall meets the roofline) All the above are irrelevant if you have already budgeted for what you want to do and you have already set your mind to it :)