With hindsight my university was woeful in preparing me (particularly in terms of relevant experience) for working life and based on friends experiences and various rumblings in the press this seems a common thread in university education in the UK
I find it bizarre that any places would be asking those kind of "describe a time..." questions up front, for me that is something that should only be done in an interview situation. As you need to have a back and forth with the candidate.
The only time I can see this being a (slightly) more valid concern is in fairly specialized industries where the isn't huge amounts of onward progression (but rather lots of people do pretty much the same thing their whole career but the dependable experienced guys get paid a lot more by the end. Based on my incredibly basic familiarity I'm thinking areas like Legal - cue a million people telling me I'm wrong...)
I'm not sure what you mean when you say they are scams - if they have a fully staffed office on the high street I'm pretty sure they're legit, plus I don't know what they'd be scamming you out of exactly?
In any area there will generally speaking be a couple of go to agencies which have relationships with the major local companies - its always worth asking friends who are in employment if they know who these agencies are. Failing that best bets are to go with national ones like Reed, Brook Street or Manpower but I suppose it depends on what you're looking for
As a final note: clearly no one has the answers and I don't think anybody in this thread claims to give anything more than well intentioned advice. Best of luck to you.