I just got on to a grad scheme for a top 100, so hopefully I can offer some advice.
The initial application isn't so bad - you've plenty of time and don't have to think on the spot. Answer the questions they ask, proof read, and you should get through the first sift. The next stage is usually psychometrics. Your success here depends on practice, rather than natural ability. I downloaded twelve SHL practice tests from assessmentday.co.uk and these helped massively.
After that, you might have a phone interview. I researched the likely questions online, and had answers prepared for them all. I kept my iPad in front of me, with the answers PDF'd. So that was OK too.
The assessment day is the hardest part, but you'd be surprised how many people don't prepare properly. Or that come across as too 'educational'. They're looking for professional individuals that will fit into their organisation. To this end, you need to be dressed well, appropriately deferential to those above you without resorting to sycophancy, and avoid obvious mistakes. Stop writing when you're told to. Ask before leaving the room. Little things.
Related to the point above regarding professionalism is the use of notes, cue cards, etc. Try and avoid this. I used no notes for the presentation, and actually saw people bringing pages of notes to the competency interview. Which is an interview about themselves!
The group exercise will be your chance to shine. Don't overestimate your opposition, and don't be intimidated. They will screw up, but be careful in remonstrating with them when they do - you don't want to come across as a bully. If they say something wrong, suggest an alternative. With my group exercise, one candidate failed to read the brief - at all, he admitted - and another asked for a vote, having been told in writing and verbally that no votes were allowed. Use names when speaking to people
Research the company. They probably won't ask you directly, but it's useful to slip snippets into the presentation, group exercise, etc. For my written report, based on the group exercise, I made sure I used an introduction, middle and conclusion. Even though that meant leaving out content. When somebody says something notable, write down the name of who said it, then use that name in the written exercise when you refer to the idea.
For what it's worth, I went to a completely non-target university. About as far from target as you could get. I had less experience than some, who had graduated two years ago and worked full time since, but the experience I did have was hard won, and carried out both on placement and during second year / final year. I had demonstrable proof of holding positions of responsibility, and absolutely no society involvement. Actual experience > societies, every day of the week. I was also a candidate that would require relocating from quite a distance away, and was chosen in preference to others living nearby, so don't let location put you off.
Hope that helps.