considering you have a corded drill for heavier work these would be very useful, and they are a lot smaller. Don't be put off by the 10.8 volts..... I use the bosch 10.8 range all the time in Studios, fantastic bits of kit.I think it's a house brand?
Generically, I've been through a LOT of cheap cordless drills and never really been happy with them until my latest purchase, which was the DeWalt 10.8V drill and driver combo set that Screwfix occasionally have on offer.
For jobs such as boarding out a loft or other carpentry stuff, the pair of devices is awesome. Add an all-in-one drill bit that does pilot, shank clearance and countersink to the drill, a decent quality set of Wera bits to the driver and it makes things quick, easy, repeatable. Using one device, you are forever changing things.
Look at battery availability and speed of charging too. Ideally, you want a minimum of n+1 batteries, where n is the number of tools and maybe n-1 chargers. This way, if it's a job that's on fot several hours and is quite intensive on the tools, you have say 2 tools with batteries in use and one on charge at all times. Ideally, you'd buy one more battery and one more charger, but we're not made of money and there's dive kit to buy too!
Look at the rated max power of the tool and the battery rating. In this case, it's a 18v, 1.5Ah battery and a 6o min charge time. Carefully, they don't tell you what the tool runs at, but if we assume 200W at 18v, then the current draw is 11A. From a 1.5Ah battery, that's about 8 minutes run time at max tilt boogie. Duty cycle of say 25% and you're talking 32 mins run time on one battery. 2 batteries included, 30 mins charge time... yeah, that's kinda ok.
DeWalt ones here: http://www.screwfix.com/p/dewalt-dck...win-pack/3292t for £120.
These have 1.3Ah batteries, run 10.8v and have a 30 min charger included. 200ish watts, 10.8v, With 3 batteries, is 18.5A, so in theory only 4.2 mins full tilt or 16 mins at 25% duty cycle. In reality, that's about right in the case of the impact driver, but the drill lasts way longer. They're about OK in terms of duty cycle. If I was doing it professionally, I'd go for the 18V deal with 3 batteries at £250, because of the longer battery life.
I'd go for one or two extra batteries on the 10.8v set at £24.99 each but the chargers are heinously expensive, like £105 each, FFS. Hence for the same money as the 10.8v set, 2 extra batteries (total 4) and an extra charger (total 2) at approx. £250 all in, you are better going for the 18v version, which has 2.0Ah batteries (200w, 18v, 11.1A, 11 mins at full tilt, nearly 45 mins at 25% duty cycle, so you don't need a 4th battery or second charger).
Entirely depends on how hard you'll work them and how long for in one sitting.
For drilling in to anything other than wood or aluminium (maybe thin gauge mild steel) you'll be wanting a corded hammer drill. Depending on what the construction of your house is like and how hardcore you want to go, I would definitely consider something with both SDSPlus and normal chucks, plus a chisel/drill/hammer selector. This will give you options.
In terms of customer service, screwfix are ACE. I had a 1.3Ah battery pack that would not charge. Took it in, they looked me up (I had binned the receipt), found the purchase, replaced it there and then with a 1.5Ah pack as the 1.3s were out of stock. Result.
In general, if you want to know about the good, bad and ugly of consumer / "Pro-Sumer" grade tools, check out a YouTube channel run by a guy called AvE. Search for BOLTR (Bored of Lame Tool Reviews).