I don't know anything about planning / building, but does the planning office not inspect sites periodically to make sure the plans are being followed?
only one in 10 council planning departments are fully staffed, with 13% operating with fewer than three-quarters of posts filled
with
80% [of local authorities] reporting they did not have enough officers to carry out their workload
because
long-term cuts to funding have had a visible impact on planning departments’ ability to retain staff... there has been a 43% fall in resources to the planning system from local authorities since 2009-10.
Well, if not properly staffed, hopefully full demolition will act as an effective self-enforced compliance to other builders in the future.
If hiring a good architect that knows the regulations well, then there shouldn't be much need for deviation from the original plans. I understand some deviations may happen, but that is when you should be in contact with the local planning department to remedy and get approval for minor changes (shouldn't be anything quite this drastic of a difference though).
I'm betting, based on how much building is going on along the Greenwich riverside, that this was approved pre-covid, then when the costs started rising afterward they thought they could cut corners to save a few bob.