You should contact your local London borough if you wish to lodge a complaint (and you never know, you might get a different result this time).
Depending on the nature of work being carried out, and the level of noise, this may fall within either s.60 of the Control of Pollution Act 1974 or possibly under Part 3 of the Environmental Protection Act 1990 if it is deemed a Statutory Nuisance
Taking the Royal Borough of Greenwich as an example, they say here:
General construction work should be restricted to the following hours:
Monday to Friday 8am to 6pm
Saturdays 8am to 1pm
Noisy work is prohibited on Sundays and bank holidays
Therefore it seems the 8am start is allowed.
As for the noise levels, independent readings will be required to see if your figure of 80db is accurate but if it is, and continuing with Greenwich as an example, it may be over the permitted levels stated at 6.3 of Greenwich's Code of Practice - assuming that the builders fall within the definition of conducting "major construction and civil engineering works".
I have not checked many other boroughs, but the above is consistent with those that I have.