That is actually what I am thinking. Most people in the UK have their stock trading account under S&S ISA account and only use general invest for the stocks are not available under ISA such as many Chinese stocks, penny stocks, etc. So, if you just have about £6k (Say) on invest you will hardly ever exceed the £12,300 gain a year.
Also, many people do not need to do a self assesment, as they are not self-employed and their source of income just comes from a single employment.
But I think it is probably worthy to keep the record of CSV file and the record of balance on the start of the tax year and the record of the balance at the end of tax years that could be used when needed.