Given you say you have Greek income, I assume you are filing tax returns to declare this or to claim the remittance basis (assuming the unremitted income is greater than £2000). If so you will need to confirm your residence position on your tax return on the residency pages. If you are not required to file a tax return then you can complete form P85 to declare that you will be leaving the UK.
UK residence is based on a tiered set of tests that consider your day count in the UK and other more subjective tests. This is known the the Statutory Residence Test and more info can be found by googling RDR3 or getting professional advice. As a rule of thumb, if you spend more than 183 days in the UK during the tax year then you are automatically UK resident for that year, if you spend less than 16 days then you are automatically non resident for that tax year, anything in between will need to be looked at in more detail. Taking unpaid leave will have no bearing on your tax position; actively working however may.
If you leave part way through the tax year either to work full time overseas or live (giving up your UK home in the process) may mean that you can split the year into a resident and non resident part. You will need to read up on RDR3 to confirm your own position or obtain professional advice.
Also be aware of the temporary non residence rules that may mean you are liable to CGT on your return to the UK on gains that arise while you are outside the UK. Also be aware that disposals of residential and commercial property remain subject to UK tax regardless of tax residence.