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...
Easyjet fly Gatwick to Zagreb, and are currently (May 5th) quoting ~200 quid (plus random rip-offs I'd imagine) for Sun either Sunday going out, and returning on Sun 31st -- cheaper fares are available, as well as pricier, depending on the day -- they don't fly Saturdays, which is a bit of a pain.
As mentioned on the DebconfTravel2011/Train page, you can look up bus times here: http://www.autobusni-kolodvor.com/en/timetable.aspx