I have a General Investment account with Interactive Investor, but I found it very painful to calculate my UK Capital Gains Tax liability on my Stocks and Shares sales.
Then someone on the ii forum suggested I try www.cgtcalculator.com and it seems to be pretty good. It seems to understand the same day rule, Section 104 rule and the 30-day rule.
If you don't know what the 30-day rule (bed-and-breakfast rule) is, then watch this video (see example 1) and here. Note selling shares of ABC on Trading 212 for instance, but then buying ABC a few days later on Interactive Investor also counts in the 30-day rule (AFAIK)!
If you have several GIA accounts, you should combine all trades into one table (CSV, XLSX, etc.) first, before calculating gains/losses and CGT.
So here are my simple steps to calculate gains/losses for CGT purposes. I also include a Trading 212 example.
Use a PC/Notebook and the browser.