1. Plane
Looking at the page at http://debconf11.debconf.org/travel.xhtml (and trying online too!), there are no useful flights to Banja Luka from the UK. Expedia says FAIL. Flying to Zagreb in Croatia and getting a bus for ~3 hours is suggested. Looking at that on Expedia, there are direct flights from LHR to ZAG from GBP250 ish, or GBP200 ish with a connection in Germany. No idea how much the bus will cost...