The Sentencing Guidelines (check them on the Sentencing Guidelines Council website) say that for a theft of over �125k the starting point is 3 years if you plead not guilty, with a range of 2-6 years. Pleading guilty right away will get you a reduction of around 1/3 of your final sentence.
As an example, say you would have gotten four years, plead guilty you will get approx 33 months and serve 16 or so then be out on licence. You might get more or less depending on your circumstances, life history, convictions and whether the judge is in a bad mood.