"Lockdown brings alarming rise in modern slavery...
Cases of sexual exploitation, which includes people held captive in brothels and coerced into prostitution, rose by a quarter in 2020 compared with the previous year. Nearly a quarter of cases involved children."
It's from the Modern Slavery and Exploitation Helpline annual assessment.
I've just read the reports covering the years 2017-2020 (those are the only ones available online).
There really doesn't seem to have been an "alarming rise" at all. The numbers they give tend to bounce up and down from year to year.
They give headline figures for "potential victims" ie people who may be victims of slavery.
The figures they give for the number of potential victims identified to them each year as a result of sexual exploitation are:-
2017 - 774
2018 - 517
2019 - 421
2020 - 481
So, yes, the number is higher than 2019 but certainly less than 2018 or 2017.
It's interesting to compare this with numbers for non-sexual exploitation (eg labour exploitation). Here, the equivalent figures are:-
2017 - 3,540
2018 - 5,258
2019 - 3,195
2020 - 1,889
There has been a massive fall in the reporting of labour exploitation in 2020, presumably due to covid.
All figures taken from the report you linked to for the years 2017-2020