If you're too ill to work, or are required to self-isolate due to COVID and cannot work from home, you may be entitled to Statutory Sick Pay (SSP).
The weekly rate for the 2021/22 tax year is £96.35 per week and eligibility is based on your average weekly earnings.
As long as you are earning at least £120 per week (or £520 per month) after your salary exchange pension deduction has been made from your pay, you will still receive the full rate of SSP.
Therefore salary exchange will not affect your entitlement to SSP unless it drops your earnings below £120 per week, and therefore only those working a few hours per week need to consider the impact on their sick pay entitlement.