It's not clear from your question whether you require latitude & longitude or OS grid references. If you know the post code of the location you can use the site below for both types of coordinates. (If you just want to specify the town centre, enter xx1 1AA - where xx is the start of the local post code. e.g. for Sheffield enter S1 1AA, for Ipswich enter IP1 1AA, etc). Enter the post code, check that 'post code' is selected from the various choices and click 'Convert': http://www.streetmap.co.uk/gridconvert.html
PS: If you don't know the localpostcode you can still use the same site. Just go here: http://www.streetmap.co.uk/
Select 'Place Name' and enter the relevant town or village. If there are several places with similar names, you will be prompted to select the correct one. Eventually a map will load with the relevant information in a blue box near to the bottom of the page.
Chris
PPS: Multimap offers a similar facility (and the page loads much faster): http://www.multimap.com