http://www.compoundchem.com/2014/03/27/ ... -uk-coins/
A very well made chart of British coinage. They should be announcing the new composition of the one pound coin soon, but I can imagine its going to be similar to a Canadian loonie (probably iron cored) since they decided to go "magnetic" with the pennies, five and ten pence.
The only coins of assured value in the UK are going to be the 20 pence for 5.0 grams, and 50 pence for 8.0 grams of cupronickel.