There are two questions in the original question.
The first is, is this the right policy for the Council to adopt and apply consistently? That's what everyone seems to be talking about in this thread.
The second is (and please note that this is a different point form the first one), is the Council within it's rights to apply this policy to all TPOs. Andy Clark is the only one so far that seems to have alluded to this version of the question. You might think I am just being my usual pedantic self but hopefully people can see that if the answer to this question is no or in fact anything other than yes then the first question doesn't matter.
The legislation says that Councils can make TPOs "If it appears to a local planning authority that it is expedient in the interests of amenity". That's all it says about the purpose of TPOs. And I mean that's ALL! The things you can't do are listed in the Act (e.g. Lopping, Topping, Cutting Down etc.). Exemptions for circumstances are listed in the Act and the Regulations (e.g. Abatement of Nuisance, Dangerous etc.). But nowhere does it say anything other than that TPOs are there to preserve amenity. The Guidelines are just, that, someone centrally interpreting what the Act means. They have no legal force. If anyone can point out anywhere in tehm that says Councils should publish and enforce policies that countermand the Act, plase tell me and I will apologise.
It follows in my somewhat linear view of this that if any works are proposed and are a subject of a TPO application, the application should be considered on the grounds of the effect on amenity, nothing else. And even then, the effect on public amenity is paramount.
What I mean is that the removal of limbs on a side of the tree that cannot be seen from any public area, as long as these works don't endanger the health of the whole tree (and therefore don't endanger the public amenity it will continue to provide) should be of no concern to the Council.
So what right does the Council have to publish the rules that it will use in determining TPO applications.
The answer is and can only be none. The Council would be within its rights to propose a policy that defines where it considers amenity to be an important issue, possibly even the species that it considers important and those that it doesn't. In the closely related world of Conservation Areas, this is exactly what happens when the Council draws up a Conservation Area Appraisal. It should consult on this proposed policy, if it doesn't it could nullify the policy. It should have due regard to representations about the proposed policy. The policy should then be approved, rejected or modified by the elected members, and then published following the appropriate Committeee hearing.
In my experience what the Council has no right to do is the following (i) redefine amenity for its own purposes (ii) create a policy that has not been consulted upon and approved by elected members and (iii) draw up a set of arbitrary rules, however well intended, about how it will deal with future TPO applications.
TPO applications are in very many respects like planning applications. If the Council decided in advance that it didn't like UPVC conservatories in south-facing back gardens and said that any future applications that had one of these in it would be rejected not only would they lose every planning appeal and then get costs awarded against them but they would soon bin the policy and fire the idiot that came up with it. Yet these tree policies appear to do just that, they are in danger of applying rules to applications that should lawfully be decided on their individual merits purely and entirely based on public amenity.
I agree with Councils that take a consistent and firm stance on tree works. But the rule shouldn't be whether the works are necessary or unnecessary, it shoudl be whether the extent of loss of amenity from the works is acceptable.
Rant over. I've had a cr*p day, sue me if I'm wrong! But I don't think I am.