'They' also bought between 4 and 6 (can't remember the exact number) 767-300's soon after they were launched but they never appeared in MON colours.
MON already have their longhaul fleet and if you study the consolidation of the big two UK charter airlines and their longhaul fleet replacements there is no need for extra longhaul capacity in the UK charter market.
Saying that 6 787's are coming in to replace the clapped out 757's and A300's in the shorthaul market where realistic capacity at the moment is 180-190 size is commercial suicide, the figures just won't stack up as the 787's will be costing about $650-$700k a month EACH more than an A320/B738.