With this fund initially buy 6 graphics cards and make a one rig and mine any altcoins with in some month you will be able to take amount invested. So go with that. There are mining calculators available in online, you will be able to get profit value you can make per day, week and month.
...where the hell can I get 6 graphic cards, that are viable for mining alts, for GBP 500,-? O.o
That's maybe 2-3 GPUs. And not accounting for mainboard, PSU, etc...
While true that alt coin mining is the only close to viable option with this budget, it's still unlikely to make you much of a profit. Unfortunately economoics of scale is key nowadays. Besides cheap electricity, of course.
If you want to take up mining alts as a hobby however, I'd recommend starting in the alt coin mining boards:
https://bitcointalk.org/index.php?board=160.0
There you should find information on what you need to get a GPU mining rig going.