Lookup field for parliamentary constituency
For interest's sake in the end I went with a solution that sat outside civicrm (for now). I used PHP to look up and return the MP name from a sql table which I created by uploading data from http://www.list-logic.co.uk/lists/parliamentary-constituencies/list-of-uk-constituencies.html

The list of people who submitted the form are being manually uploaded into civicrm daily.