Whipping cream is hard to find where I live in India. We have a dairy brand named "Amul" which sells cream of butterfat content of 25% and cannot be whipped! Whipping cream has about 35% fat content but its not available here. Can I add butter in it to increase the fat content needed for whipping, while stirring constantly for even distribution of fat?
I struggle with this answer, as it is very difficult to prove and document something is impossible.
However, there is no known good way to do this. While there are some methods to substitute a butter/cream mixture for heavy cream, they won't whip.
Adding butter to cream will not work because you're not actually increasing the fat content of the cream-portion of your mixture. Cream is made up of microscope globules of butterfat surrounded by phospholipids (membranes of fatty acids that act as emulsifiers). This is what prevents the fat in the milk from congealing into a fatty mass (i.e. butter). The butter-making process breaks down these membranes and allows the butter to form. Butter is also a water-in-oil emulsion, but you can't re-emulsify it into cream by simply adding it back to a lower-fat cream base.
An experimental solution
(This is highly experimental, so I'll remove it if someone says it doesn't work.)
If you can only get 25% homogenized cream, try freezing and then thawing it. This should undo the homogenization to some degree, so it should separate when defrosted. Let the fat rise to the top, and pour off the excess cream (or drain off the whey from the bottom) to get the total milkfat content up into the "whipping cream" range.
(This may not work. My concern is that this separate-by-freezing process won't actually give you whey + cream… for the same reason cream + butter won't work. The emulsion may be destroyed by the separate-by-freezing process. It may indeed create a higher-fat solution… but the resulting concoction may not be "whippable". Try it and report back. Thanks.)
When cream is heated to between 115 degrees Farenheit and 145 degrees Farenheit, more fat becomes available to separate from the buttermilk. If you have a candy thermometer, I'd try slowly heating the cream to within this range, then cooling it so that the cream can separate from the buttermilk, then trying to use that to whip, as I suspect it will have a higher fat content to buttermilk ratio.
As well, double cream is made by putting cream into a centrifuge and forcing the buttermilk out through the physical action of separation. It's not making butter, but a step in between. So if you have access to a centrifuge (probably not I'd say?!) you could always try that....
Actually, forget what I just said - try this! It looks like you CAN add butter to milk to make whipping cream (but I'd use unsalted butter): http://www.wikihow.com/Make-Heavy-Cream
Around fifty years ago we had a device that would make cream from unsalted butter and milk. It was a more modern version of this. I suspect that the result might not whip.
A quick look on ebay UK found an 'antique' one for sale. I'm not aware of any currently manufactured.