With the USD:GBP exchange rate in addition to the fact it's a bank holiday weekend in the U.K., nothing is going to be cheap. OK and BA are probably your only options ex-LHR.
You can go by Easyjet out of LGW or STN for GBP 125 or by Ryanair from STN to Brno (maybe more, maybe less) but there's the cost (and hassle) of getting there.