The Makita that Nightglow linked is a really good drill, Li-Ion battery and a fast charger so no real need for two batteries. Screwfix price on that is hard to beat at the moment. I recently picked up a DeWalt equivalent with two batteries and a fast charger, I think that was about £150 from Screwfix, extremely happy with it.
If you do fancy the Makita drill Nightglow posted I just came across it on the B&Q site with 2 batteries for an extra £30, didn't check the Screwfix site to see if they had it or not with an extra battery.
I know your budget is £100 and the extra battery puts you £30 over and of course like others said one might suit you fine but it's nice to see the options.
Makita LXT 18v with 2no. 1.3ah batteries.
And with 20 years using of Makita, I've never needed to use the warranty.
Any reason for the preference of the 18v Makita vs the 24v Erbauer for £20 more? I've had good experiences with both brands before.
Link here: http://www.screwfix.com/p/erbauer-erf395com-24v-2-0ah-ni-cd-cordless-combi-drill/48544
Erbauer are hit and miss, but mostly miss. They are basically just sourced from china and re branded with a German sounding name. I have also heard that it's almost impossible to source spares for the tools.
I've got this and been very happy with it:
http://www.screwfix.com/p/dewalt-dc100ka-gb-18v-1-3ah-ni-cd-cordless-combi-drill/73596
The batteries are NiCd rather than Li-Ion, so they are physically bigger but they are more robust, particularly if you're going long periods without using it.
The reverse is actually true. Li-Ion are far better batteries. Lighter, charge faster and do not have a memory effect. Keep topping up the charge on a Ni-Cd battery and it will kill it. Plus, Ni-Cd batteries are notorious for self-discharging. NiCd you charge before use, LiIon you charge after and its ready for when you next need to use it.