The true extent of underpayment is very difficult to estimate, but a range of 1 to 2 million underpaid workers is likely, or between 4% and 9% of employees aged 25 and above.

This is according to the February 2018 briefing paper (pdf) from the House of Commons Library, which looks at what is known about workers paid less than the minimum wage. 

The paper finds:

HMRC found 98,150 underpaid workers in 2016/17. This was an increase of 69% from 2015/16, and the largest annual total on record
Data from the LFS indicates that 77% of those who usually do one or more hours of unpaid work per week are not paid a fixed hourly rate. In other words, most unpaid time is done by employees on a salary
For the same reason, ASHE does not capture the informal economy, for example, work paid cash in hand. Informal work is likely to have a higher rate of non-compliance with the minimum wage.