I would go with white plastic oval conduit. IIRC two off 2.5mm2 T&E will fit up a single 25mm oval. However, the knockouts in most back boxes are only 20mm so you'll need to open up the holes to get the conduit into the box - which is important if you want to be able to get the cables in easily afterwards.So 2 x 20mm conduits to each backbox is the way to go?
For lighting, it really depends on how many switches/cables are going to be needed. For a single 1mm2 drop from ceiling rose to switch, a 16mm oval will be more than enough. Im my mates bungalow, some of the light switch drops are the widest oval (32mm IIRC) to cater for multiple switches, some of them 2-way so needing a second (3 core + E) cable to the other switch.
I'll second the comment about using deepish boxes. Too many people put in shallow boxes and then it can be problematic getting the cables in safely - especially with low profile fittings like the screwless ones my mate used, or deep fittings like the dimmers he's used, or worse still - low profile fittings with deep dimmers in them.