These statistics only represent the cases that have been reported to the FMU, and where the FMU has actively given advice or support. Forced marriage is a hidden crime, and these figures will not reflect the full scale of the abuse.
To better understand the prevalence of forced marriage in England and Wales, the Home Office is considering the findings of the feasibility study it commissioned in 2024. This was conducted by the University of Birmingham and University of Nottingham, and it explored how accurate estimations of the prevalence of forced marriage and FGM in England and Wales might be made.
Full info at https://www.gov.uk/government/statistics/forced-marriage-unit-statistics-2024/forced-marriage-unit-statistics-2024
But good that they acknowledge that these are the ones they know about, and can only guess how many others there are.