What is the value of pre 1947 silver coins?

I'm assuming that you mean British and not US Coins.

British silver coins dated 1919 and before are sterling (92.5% silver)

While British silver coins dated 1920-1946 are 50% silver.

No British coin has been struck for circulation in silver dated 1947-present.

For the sterling coins:

Threepence - 0.0420 of a troy ounce
Sixpence - 0.0895
Shilling - .1682
Florin -.3364
Half-Crown - .4206
Crown - .8409

For the 50% silver coins

Threepence - .0227
Sixpence - .0455
Shilling - .0909
Florin - .1818
Half-Crown - .2273

In order to calculate the current values you need to multiply the silver price by the silver content. For example if silver was valued at $30 an oz and you had a 50% silver threepence, you would take 30 times .0227 which gives you a value of about 68 cents.