The Ukrainian visa schemes give applicants a visa that is valid for six months and only one entry to the UK. If you are intending to stay in the UK for a period longer than this you will need to get a Biometric Residence Permit (BRP), the BRP will allow you to stay in the UK for three years and to leave and reenter the UK.
Depending on how you applied for a UK visa, you may have already automatically applied for the BRP. Check your "Permission to Travel" letter and you should find this information there.
If you used the UK Immigration ID Check app or if you visited a visa centre to register your biometric data, your Biometric Residence Permit will be issued automatically. You will have received instructions about where you can pick up the BRP, usually from a nearby post office.
If you have not had a BRP automatically issued then you will need to apply for one using this link.