Uk Plates (Copied info from the site):
As usual, using "Vehicle Enquiry" from www.vehiclelicence.gov.uk/EvlPortalApp/index.jsp
It is also possible to guess an approximate year from first or last letter, but I do not know how.

These pages help with the identification of the years:
www.ukcar.com/features/reg_year.htm
www.dvla.gov.uk/vehicles/regmarks/reg_marks_memorytags.htm