What Starmer paid for the land would be in addition to his capital gain. The land was sold for an estimated £400,000 although partly owned by his father's estate;
https://www.bbc.co.uk/news/uk-politics-68322058
Keir Starmer paid £99,400 in UK tax on £404,000 income, Labour reveals
https://www.theguardian.com/politics/2024/feb/16/keir-starmer-paid-99400-in-uk-tax-on-404000-of-income-labour-reveals
Earlier tax years here:
The Labour leader's records show the total UK tax he paid was:
£51,547 on total earnings of £147,633 in 2020/21
£67,033 on total earnings of £212,087 in 2021/22
His earnings [for 2021/22] included more than £85,000 in capital gains from the sale of a home he had bought with his sister, which she and her children had lived in.
https://www.bbc.co.uk/news/uk-politics-65052473
In addition, his parents' home was sold for £455,000 in October 2020 - he would have been entitled to a quarter share of that property.