If in the UK you can buy physical silver (coins or bars) from the royal mint or royal mint authorised dealers. Just search the internet. You probably want to buy whatever is captial gains tax exempt (I think silver britannias are). Shop around for the best price and also be careful about VAT as I think you may have to pay that on some purchases and not on others . Of course when you buy physical silver in UK as an investment you also have to consider what GBP/ USD is up to as any change in exchange rate in future will affect your return (as silver is priced in USD).
If you don't want physical then you could buy shares in a silver ETF (eg. SSLN) which is backed by physical metal or if you want to spread bet it you could buy spot silver.
It all depends why you want to buy it. If just as an investment then you get a better return buying etf or spread betting (easier to cash in when you want to and potentially less hassles with tax). If you are worried the banking system will freeze up and you can't get money out at some point in the future ( possibly not likely but you can never rule it out) then it's physical - stored outside the banking system either with a company or stored somewhere by yourself.
I'm not an expert on this but have just read a bit about it lately as lots of articles suggesting silver will go higher over the next months / year.
