Not that bad

Derby is a VEEEERY small city but you can find almkst everything. The nightlife is not that bad but there are few places so if you find the right people you will usually go out in close cities, during the weekend, such as nottingham, Birmingham or places a little bit more farther like Manchester. 
In my experience i was lucky that I met the right people to go out with and I made soooo many friends, so even if the city doesn't offer that many things at the ends poeole, accomodation and house parties, clubs and stuff like that got my 5-months experience good!

The city, as the whole Uk, is a little bit expensive But you can easily find a little job to get some money!

the food is rubbish in the 95% places where you go but I'm Italian so you probably would like foods over there??

i was there for the master (after bachelor) and the modules wasn't that easy but no even that difficult (probably because of my good background in Italy) so you need to study at least a little bit to pass.

i got 92-93-54 in my 3 modules (range of marks from 40 to 100) that are 30/30/20 in the Italian sistem (from18 to 30) 

I would recommend derby however as Erasmus as pace even if London or Nottingham are better