If you'd rather pay someone, I imagine Hyperoptic are the best bet for a live product. But you will indeed need the blessing of the freeholder. However, with that many flats, there is likely to be a management company of some sort - either one made up of some of the residents, or one engaged by the freeholder.
In enlightened times, the freehold might even be held by a resident's management company!
An alternative with BT is just about becoming possible; they are about to start their work on "FTTB" (fibre to the basement), which would then make use of the existing telephone wires to get service throughout the building - much less disruption to the fabric of the building, so much more acceptable to management companies.
According to some BT Wholesale documents, the FTTB field trials are in the roadmap to start in the Apr-Jun quarter.
I imagine the solution would be VDSL2 for now, with the potential to become G.fast later.
1) Does anyone know the laws or requirements of doing this, since in essence I would be becoming an ISP. Any resources surrounding the legalities or things like data retention would be appreciated
2) Can anyone provide me with any general guidance or advice regarding my plans ?
I haven't heard of anyone doing this around here, but a lot of places that want to retrofit a solution into apartments do so with FTTB: fibre into the basement, then copper through the building. In places life Korea, the copper part is often cat 5 ethernet, but they might be considering G.hn.
Australia's NBN seems to be planning to re-use the telephone copper for FTTB, alongside vectored VDSL2.
Off the top of my head, I suspect you'll have serious issues with fire regulations - with the need to use the right kind of cable (low smoke), and the need to not leaving any holes between floors that can carry fire or smoke. This would probably make freeholders wary.
However, in the rural space, you can see the people at B4RN who are digging in their own fibre across fields and into homes.
Their concerns about being a perpetual blind-spot to BT means they have set up their own community-owned company (to make sure BT can't take it over), and are doing most of the digging through farmer's fields, with free wayleaves and (nearly) free volunteer labour.
Making it commercially viable requires a lot of take-up, and a lot of community involvement.
Their website and business plans might help you: