what is 200mg of caffeine?

Anna

Before discovering I'm pregnant I drank 1-2 double espressos a day. I know this is quite a lot and it results in horrendous headaches and moods if I ever try to stop Cole tell. Since finding out I'm pregnant I've tried to switch to green tea which works if I have the mental and physical capacity to manage the headaches but if I want a coffee how can you know what 200mg is?! If i have one espresso a day is that too much? Do I have loads of caffeine room? I have literally no clue how to work it out.... 

answers in U.K. Measures if poss please