You can enter this code directly into the browser console on the Financial Times' page on full constituency results for the 2017 UK General Election to get a csv-compatible output for parties who held the seats in 2015 and 2017.
You can see the results for this code in this Gist, in the file output.csv.
You can read about what this is at my first blog post on the UK General Election results for 2017 -- that explains how I created this code and why.