OK basically the only free-server your gonna get is your own.
Some clans `run` public servers like [Ghosts] run gamesdomain public servers and they are then intitled to play on these servers, but they are more caretakers than owners.
An 8 player private server is only £35 per month so surely between 8 of you this is not that much m8.
http://www.warp-core.co.uk/site/main.php?nav=prices
Theres probably cheaper server providers out there, but this is all I could find right now.
To make that 8 player server `public` you are looking at around £60 per month, so go for a passworded private one.
p