OZZIEPOST
In the UK yes all congs are charities in their own right, with a model constitution from the Society, this came about because of a tightening in the UK of the powers of the Government Body The Charity Commission, following several frauds on charities which were not regulated/registered. The Commission could, at will, take over the running of charities which were negligent or financially unsound.
If congs had not registered then they would have had problems with any form of fund raising (including collection boxes) and owning property, as the Charity Commission would have probed transactions deeply.
The Society chose not to make the congs part of one charity (the IBSA) as this would expose the branch to clear responsibility for what happrens in congs. It also removed at a stroke some probable legal responsibility the branch had for congs - leaving the local elders (who are all trustees of the charity) responsible for any and all legal actions against the cong/ charity.
Power was not lost the the Society as the Constitution strictly controlled what happens to any funds in the case of cong closing/merging or whatever.
Most elders are blissfully unaware of the legal responsibilities they hold and the consequences for them if any action is taken against the cong.
At about the same time congs were asked to pass Resolutions allowing the Society (the IBSA or new Watch Tower Society of Britain) to use donated funds in any way and any part of the world it wished. The Charities Laws do not allow charities to spend money on anything other than the aims of the specific charity so special provisions (the Resolutions of Approval) were passed to approve expenditure outside the scope (outside the UK) of the Charities Commission - so money donated for the British KH Fund, for example, could be spend in Africa or indeed anywhere.
Registering as individual Charities for every congregation allowed the Society greater, not lesser control over the congs as it is now written into cong Constitutions that the Societys can at will appoint or delete elders etc. Other provisions exist which give the society control of cong property/funds.
The situation is unique to the UK (England and Wales) with different provisions in Scotland, the Isle of Man is not covered by UK Charity Law even though it is administered by the british WT branch.