If you sell me £8 more of stuff, and I sell you £2 more of stuff, then it's true that overall our trade has increased £10.

Worth thinking about the detail, though, because it's the same as the announced Japan-UK trade deal. Under it, Japan will see 80% of the increase in trade.

In other words, the figure of £15.2 billion in the UK Government's press release isn't expected growth in UK exports - it's total growth in trade between the UK and Japan. But the Government admits elsewhere that 80% of the growth will be on Japan's side. https://www.gov.uk/government/news/uk-and-japan-agree-historic-free-trade-agreement

I should also add that even sites like the FT, which have produced superb analyses of the deal, seem to have missed this nuance, because it's not obvious from the press release. It has to be read in conjunction with the 13 May Policy Paper.
https://www.gov.uk/government/publications/uks-approach-to-negotiating-a-free-trade-agreement-with-japan/uk-japan-free-trade-agreement-the-uks-strategic-approach