I was a bit uncertain on this ... however, having spoke to my accountant today, I decided to do some further research:
If you're a basic rate tax payer, there is no difference in the tax that you pay on the dividends, if your shares are in an ISA or not - the dividends are taxed at source, at a rate of 10%. The only advantage is to higher rate tax payers. If the shares are held in an ISA, then the tax payable remains at 10%, ; whereas if they are not in an ISA, the tax payable rockets up, viz: 32.5% if you're a higher rate taxpayer and 37.5% if you're an additional rate taxpayer.
Taken from:
http://www.which.co.uk/money/savings-and-investments/guides/stocks-and-shares-isas-explained/what-is-a-stocks-and-shares-isa/
I also rang my bank today to ask about transferring my shares in to an ISA and one of the questions I was asked was, if the shares were company shares. I answered, "no" to the question, so I didn't glean any further information from this, but I'm wondering if there are additional rules regarding this .... why else would they ask ??