As part of the immigration process, many individuals are subject to pay an Immigration Healthcare Surcharge (IHS). Where such individuals find themselves in employment and income taxes have been paid, there should be a rebate payable against the initial IHS at the end of each financial year.
You may be interested in these active petitions
Personal note: Whilst I don't disagree with the principle of an Immigration Healthcare Surcharge and helping to cover costs / fund the NHS - I do believe without a rebate system in place, the IHS unfairly 'double-dips' from the pockets of immigrants that end up working and contributing to the NHS through income taxes.
We need a fairer system - one that protects the NHS without unfairly discouraging immigrants from coming to the UK to work
https://www.gov.uk/healthcare-immigration-application