What is the sum of total pounds of purchase in year 2018 for all branches in London?
SELECT sum(total_pounds) FROM purchase AS T1 JOIN branch AS T2 ON T1.branch_id  =  T2.branch_id WHERE T2.city  =  'London' AND T1.year  =  2018
SELECT sum(total_pounds) FROM purchase AS T1 JOIN branch AS T2 ON T1.branch_id  =  T2.branch_id WHERE T2.city  =  'London' AND T1.year  =  2018
[{'sum(total_pounds)': 3531.5899999999997}]