Ordinary shares (Details)	6 Months Ended
	Jun. 30, 2019 Vote £ / shares shares	Dec. 31, 2018 £ / shares shares
Ordinary shares
Votes per share | Vote	1
Ordinary shares, shares authorized | shares	31,995,653	15,452,420
Ordinary shares, nominal value | £ / shares	£ 0.01	£ 0.01