Brexit, Uber, Twitter: Everything that matters this morning

Theresa May signs Brexit letter
The PM has signed the letter that will formally begin the UK’s departure from the European Union and it will be delivered to European Council president Donald Tusk today. In a Commons statement the prime minister told MPs this marks “the moment for the country to come together”.
The future for brands in the UK is unclear. Some industry leaders say advertising is key to the UK’s success post-Brexit while negative effects of Brexit make headl