Look into custom in ear plugs. There are many levels of companies to do this. You can double up your current solution

Cheap and DIY: Decibullz or Radians

Next level up: Something like "Big ear inc" or alternative, I bet there is something for the UK

Maximum: Go to an audiologist (or ear doctor and ask for a referral) and get custom molded ear plugs and explain you want maximum sound reduction.

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Or involve the authorities, your apartment complex / building you shouldn't have to live like that