Likelihood of getting planning for granny annexe

Their main concern in this regard is that the granny flat will end up being rented out, even if it's after the relative has passed away. They want the granny flats to be connected to the main house and with doors between them, and the main condition of planning will be that once the granny flat is no longer needed, the space is converted into part of the main house as extra living space/bedroom for the main house, and is not rented out as a separate liveable space.

Sheds in people's back gardens aren't the solution to the housing crisis, as they end up being sub-standard and causing different issues with regards parking, drainage, garden space, noise etc.

If you genuinely need a granny flat for a relative, there are provisions to allow that but it needs to be designed in accordance with those rules, unless there's a significant reason why it can't. You should organise a pre-planning meeting, or engage an architect/engineer/surveyor to advise.

You can have a separate entrance to the granny flat, however you still need some sort of link between the house and the granny flat.

As for not wanting the granny flat attached to the house because it would affect your link between the kitchen/living and patio area, the planner is simply going to deem the family flat as not being necessary if that's why you don't want it attached to the house.

Family flats are granted on the basis that they're deemed necessary, and as such issues like attaching it to the house aren't meant to be that big an issue if the family flat was truly needed. The idea is that you construct the family flat against the house, and when the time comes that it's no longer required it becomes part of the main house (eg. you convert it into a house extension).

Unless you can prove an actual reason why it can't be connected to the main house (the only such reason that might come to mind imo is if the site layout doesn't allow it, but it would have to be an extremely poorly laid out site), the planner isn't going to accept reasons like it will devalue your own home or affect your use of the patio. Your basis for needing a family flat to begin with are deemed to be more important than those issues.

Again though, it doesn't help matters, as it causes a multitude of other issues, and there are plenty of shocking cases out there where people have done such structures, rented them out, and they're incredibly below standards.

If your site is big enough for another dwelling, that can be reviewed. But if your basis for needing the structure is as a family flat, there are particular things it must comply with.

You are quite wrong. Sheds in gardens could play a huge role and this obsession with perfection is leading cause of the housing crisis

It is one example. From NZEB to onerous spatial requirements, size limits to development levies, we really are not treating the Housing Crisis as an actual crisis like Covid or the Emergency. The West has gone deep on the social democrat housing codology you are espousing, and from San Francisco to Berlin we are suffering the consequences. But I agree, overregulation isn't the leading cause. Urbanisation is the key, and Central Bank polices are factor too but over regulation is still a substantial factor.

You won't get permission for a detached granny flat as it would be considered a separate house.