Perhaps depends on where in the UK you are based, but how about London?... trip on the Thames, plenty of sights, London Eye, Covent Garden... As the price would be cheaper than going abroad there could be a range of pre-booked 'treats' in with the price - One of the many ghost walks, a meal at Hard Rock Cafe and a West End show would keep them entertained in the evenings? ... although not sure of the Olympics timings... maybe that would be a plus point though!? Heard 'euro-study-tours' are decent, not used them personally though. Good luck!