Think before you jump
It would be better if there was a clear accepted concept of what is to be achieved, before proceeding to legislation. Otherwise the EU judges will take them apart again.
We accept that GCHQ has full coverage, because they are above the law.
So we syphon everything to GCHQ (US Branch) within the first few days.
They provide to police as needed, as long as it suits GCHQ and friends (filter out plants, etc).
No duplication, no data loss, no ISP inconvenience, but not admissible as evidence.
We make data available to police as needed.
They make it available to GCHQ and friends above the law (warn plants, etc) within the frozen period.
After, they can still make it available to police as requested.
No duplication, no data loss, no ISP inconvenience, frozen records admissible as evidence.
Both concepts assume that the police and the secret services work well together, to avoid data gaps and save money.