'Operating at up to FL100'
You might get through at a min level of FL80 but certainly no lower, in fact higher might be preferable.
I've know TC descend traffic (very rarely) and ask Farnborough to work it west of Heathrow in order to avoid congestion, in fact the original plan when they extended the LTMA westwards back in the early '80s was for Farnborough to work these 'low and slows' but my boss at the time, non operational at Farnborough and ex Heathrow, said we hadn't got the staff.
Of course we had, but he didn't bother to ask us operational types if it was feasible.