mAh is fine if you're measuring like for like, voltage-wise. Two NiMh AA cells, two Li-ion cells either singly or in parallel. Wh is definitely a better way to measure capacity like that, but battery manufacturers tend to quote mAh, so that's what people are used to. Some LiPo manufactures seem to advertise based on Wh, mind you. I guess different chemistries/voltages don't tend to be able to be used in the same equipment interchangeably, so the issue doesn't crop up much.
Is it really a measure of capacity? Without the voltage isn't it fairly meaningless? If you know the voltage wouldn't you multiply it by the current and give capacity in Watt hours?
mAh is a measure of capacity, so unless you forget to pay the electricity bill this is unlimited mAh.
The quality/safety of some similar chargers is suspect. I'd like to think that Argos selling it under their own brand name means it has been tested in some way. I hope I'm not being naive! Hot.
One comment says 2.1A