SOP in the London TMA. The LAM arrivals are all cleared to FL150 level by Saber and if the world then went 'pop' they would all enter the Lam hold at FL150. The same applies to the other 3 arrival stacks. It is mitigated by the LAM etc stack controller descending them to a proceedurely safe level before the hold is entered. If things start to get a bit 'hairy' and and levels up to FL150 are either not being cleared quickly enough or LL APC are unable to take sufficient from the said stack the 'standing agreement' of FL150 must be ammended.