You will have to complete tax returns for each year in question, calculating the taxable element of your RSUs and declare them on the returns together with all other income and claims for tax credits etc.
It is likely that you will have to pay the late filing surcharges 10% of tax when return is not submitted within 2 months after filing deadline.
If you do file your returns voluntarily, penalties may well be mitigated, and interest may or may not not be charged on late payment at 0.0273% per day or part of day.