The Water Act 2003 is a total bore but if I read it correctly, it says everybody is right:
Subject to the following provisions of this Chapter and to any drought order or drought permit under Chapter 3 of this Part, no person shall—
(a)begin, or cause or permit any other person to begin, to construct or alter any impounding works at any point in any inland waters which are not discrete waters; or
(b)cause or permit the flow of any inland waters which are not discrete waters to be obstructed or impeded at any point by means of impounding works,
In other words a Water Butt is potentially illegal, except:
The restriction on abstraction shall not apply to any abstraction of a quantity of water not exceeding twenty cubic metres in any period of twenty-four hours, if the abstraction does not form part of a continuous operation, or of a series of operations, by which a quantity of water which, in aggregate, is more than twenty cubic metres is abstracted during the period.
If more than 20 tons a day is required, a license is needed.
Except, this can be varied by order of the Secretary of State. (Usually to deal with droughts.)
So, in the UK, although you do not own the water that lands on your roof, you have rights to it provided you don't go bonkers and take more than 20 tons a day. The legislation is aimed at large land-holders who might interfere with public water supply, not water butts in a small garden, unless the Secretary of State has reason to interfere, which is unlikely. The Secretary of State can also limit the amount of water a house-holder can take from the tap, even if it is metered. I remember a garden-proud pensioner from Devon going to jail in 1976 for repeatedly using a hose during a ban.
Dave