either you find a solicitor (expensive) or you do it yourself. a lot will depend on what's involved in your individual situation. are there property issues? child issues? etc...
if you think you can agree on everything, I'd say do it yourself. I did mine and it cost about �300. use this link
http://www.hmcourts-service.gov.uk/infoabout/d ivorce/index.htm and get the information pack with the forms you'll need.
if you need a solicitor.. the sky's the limit! as a starting point I'd say at
least �1000.