ATC can only withold clearances for traffic reasons. Over and above that they can warn you that, in their opinion, something you intend to do is potentially dangerous or contrary to the ANO, and then simply say "There is no traffic reason to affect you . . . ."
However, bear in mind that ATC may be passing on instructions from the aerodrome authority and they, without getting into a debate about ordinary and public aerodrome licences, can have a bigger say on who does what to their aerodrome. ATC should state if it is passing on such instructions.
All this is a bit academic. Any airline thinking of flying a B777 anywhere would have done the number crunching before turning up, unless it is a completely unexpected diversion.
We have B777s into Luton and they will often take an intersection departure rather than the backtrack.