There is no real "norm" because the hard drive can be from 160 GB (typical minimum) to 1 TB (1000 GB), and RAM can be 2 GB to 8 GB (generally the more RAM the better - Windows 7 will work with 2 GB but more RAM improves performance).
Intel i3, i5, i7 processors have positive comments speed wise, but given type of usage pretty much any CPU should be OK, and you may see a 'clock speed' of 1.9 GHz up to 3 GHz (faster often better, but the CPU could be 'dual core' which means it has two processing units). Sometimes the choice of CPU (for hard core games enthusiasts) may add 100 or more to the cost of the machine, and is not good value for money for day-to-day usage.
Pricing can be 200 to 500, depending on what deals are on offer and whether a place expects a new line (eg Asda reduced prices by 100 quid on some laptops, but similar can be found from Currys/PC World and others). Worth looking on ww.HotUKdeals.com and do a search for "laptop" (do note that some systems may come with no copy of Windows - not very likely but not impossible, so check for that being listed on any web site before considering purchase).
Also check if JohnLewis.com has the same or similar laptop, because they often have 2 or 3 year warranty which could be handy. Not so obvious tips include not using the laptop on a bed/duvet as it may block the air cooling and the laptop can overheat and die... (just that teens seem more likely to do so if they disappear off to a bedroom to chat with friends).