Please include where I can buy the dataset and how much it costs?
In the future you should be able to use the LLPG (local Land and Property Gazetteer), which would return a British National Grid coordinate. This can be easily converted to Lat/Long and I would argue it is probably more up to date than the OS AddressPoint data, given that it is updated by the councils who create the address and updated daily in most cases. However I am not sure if this is available yet, it has been hampered due to licensing restrictions from OS and Royal Mail.
The most accurate way to do this is to use the Ordnance Survey AddressPoint dataset. Their price list seems to suggest that for use on just one terminal costs around £16,000.
However, if you are willing to sacrifice some accuracy you can do this for free at http://www.nearby.org.uk/coord.cgi using the postcode part of the address. They also have an API which would allow you to automate this. If you have an address without a postcode then you can obtain the postcode from Royal Mail (see http://postcode.royalmail.com/), although I think they only let you lookup a limited number of addresses per day. Also, the Ordnance Survey have released their CodePoint dataset as part of the OS OpenData initiative - available to download from https://www.ordnancesurvey.co.uk/opendatadownload/products.html. Of course, if you want to represent an address as a point then all the postcode method will provide you with is the location of the centre of the postcode area.
OpenAddresses.org provides a free (beta) service where users can create/edit/upload addresses. Unfortunately, the coverage in the UK still appears to be pretty spotty.
Don't forget that in the UK Post Codes are generally small (25-50 houses) so a post code based system may be acceptably accurate.