Your SA302 (also called a Tax Calculation or Tax Computation) shows your income for that particular year. It also includes National Insurance and tax contributions at you marginal rate. Your SA302’s are available via your self-assessment tax portal on the HMRC Revenue and Customs website. You will need your UTR (unique taxpayer reference) to access this.
If you use an accountant, then these should be readily available via the accountant. Remember, the lender needs the SA302’s to be less than 18 months old. This means the 5th of October marks the time of the year that the current year’s tax return must be submitted if you want to use the income for a mortgage. HMRC give you until January of course so your hand may be somewhat forced by the situation..