Switzerland and the USA also have coins with constant weight to value ratio:

Swiss: 1/2 Fr = 2.2 g; 1 Fr = 4.4 g and 2 Fr = 8.8 g.

USA: 10 c = 2,27 g, 25 c = 5,67 g and 50 c = 11.34 g

These constant ratios are the remains of earlier coins made of silver which determined the value. In such a case the weight to value ratio must be constant. For the UK there also is a remainder of this (5 and 10 p coins). To use a constant ratio for copper was decided for the decimalization. The Euro coins were newly designed and do not have such a constant ratio. To illustrate the weight to value ratio for coins in circulation I made some graphs that show the weight for a unit (10 c/p for copper and 1 Euro/GBP/USD or SFr for "silver") made up of each individual denomination. E.g. (for GBP) the weigth of 10 1p coins, 5 2p coins and 20 5p coins, 10 10p coins, 5 20 p coins, 2 50p coins, 1 1P coin and half a 2P coin).

It is interesting to see that the UK coins are relatively heavy compared with US and especially swiss coins.