Can't Identity Composition Of This 1972 UK 25 Pence Anniversary Coin

From the photo and the general color and finish of the coin this looks like the cupro-nickel version. Hondo Boguss is correct: the silver version was only issued as a proof and has a mirror-like surface. Even an impaired proof should look whiter than the cupro-nickel coins which may be very slightly yellowish.

To be sure you could always try the tissue test.

There is a guaranteed way of telling the .925 silver crown from the cupro-nickel version. I had to check the coins in my collection first.

The edge milling on the silver version is coarser than that on the cupro-nickel version. This photo shows my proof silver coin (still in its mylar sealed envelope of issue) with a cupro-nickel coin underneath.

If you find it hard to tell from these images, you could compare the edge of the 1972 25p coin to other British coins - if it's silver, it should be like the milling on the 1887-92 Victorian Jubilee Head crowns, and if it's cupro-nickel it should be exactly the same as the milling on the 1965 Churchill, 1977 Jubilee, 1980 Queen Mother or 1981 Royal Wedding crowns.

All cupro-nickel coins issued from 1947 had finer milling than the older silver coins: I assume this was done to enable bank clerks to separate the silver coins when stacking them.