ESPP Taxable Gain

I have recently enrolled in my company's ESPP and I have been taxed on the gain I have made at the purchase (I got an employee discount). I have been looking on internet what is the taxation applied in UK but I cannot find it. I have been taxed 85% and it seems a bit weird, if not unfair. Would it be possible to have an explanation for dummies?

Hi,
Please refer to: Tax and Employee Share Schemes.
Thank you.