I don't really know much about how agents select from showcases I'm afraid. They will be looking in a different way from how they do when you're younger.
Finding the right agent is also very much about 'fit'. Do they have a DD shaped space on their books? Do they focus mainly on stage, or mainly on screen? Do you click when you meet them and are your needs and their offer compatible? Some agents have a more managerial style, others less so. On the whole you hear from them when you have auditions or are booking, but otherwise they generally have dozens on their books and time spent chatting/hand holding clients is time not spent subbing or pushing to get you seen. They are not your therapist or coach!
To get into good agencies outside the graduate shows, you need to fill a gap on the books and be able to show a track record of booking work (agents need to earn a living and the best way to show that you will make them money is to already have made money - another fun catch22 in the industry).
It's a miserable business finding one on the whole - unless you happen to have just been cast in the latest international blockbuster! Happily for graduates, plenty do attend showcases and so you have a captive audience rather than being the 200th representation request email in the inbox that week (yep, that is what the desirable agencies get every week).
The absolute top agencies - 42, United, Independent, Curtis Brown etc probably only take maybe 10 graduates on a year between them, and that's across all the different colleges. Those alumni pages are well worth looking through to see who is taking on from which colleges and whether they seem to have a 'type'.
It is well worth spending the time at college getting your head round the different agencies and who and what they rep. The best place to start is probably looking at the PMA list for who their members are. Then start working through individual agency websites to see what is what.
Look at West End casting lists - google the cast, who are they repped by etc. There's no short cuts but it will be very useful.
If you are a 19 year old with red curls, there is no point applying to an agency who already has two of those. If you are a blonde mezzo who fits a certain role type, apply to the agent that doesn't seem to have one. You need to look for the gaps that you fit.
Yes it will be weeks of work, but well worth the effort.
Write and invite them to attend things that you are in. Short letters that set out all the information very clearly and with a couple of weeks notice.
Make sure you have really good headshots ready for early Spring of final year (not the god awful ones that a lot of colleges seem to think are appropriate) - you may need to book 3-4 months in advance for the popular photographers. Again make sure that they are experienced headshot photographers and not model ones or portrait ones. It is a very specific art.