The Bank Holiday is on the 28th (it's always the last Monday in May). In theory there shouldn't really be too much of a problem getting a reasonable fare for the proposed dates - out mid-week, back on the Saturday or Sunday of a Bank Holiday week-end (most week-enders would be returning on the Monday), but of course it's rather short notice.

If you want to fly out from LHR but don't mind which airport you're flying back into then Czech Airlines may be your best bet as they fly from both LHR and STN. I got quite a good deal out of STN a few weeks ago.

As others have said fares from the UK are never going to be very cheap because of taxes... the only way to avoid those is to go by train but I doubt that will work out much cheaper.