'You berk' is an insult you may hear in the UK. It means 'stupid person', 'maker of errors'. 'Berk', which may also be spelt 'burk', is pronounced with the vowel sound of 'fern' or 'term', IPA: /bɜːk/.
The insult is not felt to be very offensive, although in its original form it was. It is rhyming slang. The 'surface phrase' is 'Berkshire (or Berkeley) Hunt'. In British English the first syllable of 'Berkshire' and 'Berkeley' is that of 'bark', IPA: /bɑːk/. (A similar feature of pronunciation, with the spelling '-er-' representing the sound 'AHr', can be seen in clerk, the proper noun Hervey and the military rank of 'sergeant'.) In American English, however, 'Berkeley' (the city in California) is pronounced IPA: /'bɜːk lɪ/.