The UK is currently relaxing its domestic 'Lockdown' restrictions' in several phases from now until the end of June. However, international travel regulations have not yet been made looser and have changed once again. The government's website has been updated providing details of the current process in one place for international travel from and to the UK (it is written from the perspective of a UK resident). Stricter restrictions apply if you are entering the UK from one of the 'Red List' countries (including allowing entry to the UK only of UK citizens (including dual nationals) and UK residents). At the time of writing Serbia is not on this list.
A key change is that people leaving to the UK must complete a new form to justify the reason for international travel supported by evidence for the reason. A copy of this form must be kept with you at all times whilst travelling from the UK (otherwise an 'on the spot' fine may be payable).
The list of permitted reasons includes work, volunteering, education, medical or compassionate grounds and weddings, funerals & related events. The complete list of permitted reasons can be found here.
The guidance is also written from the point of view of a UK resident. As returning home is not a stated reason for international travel from the UK, we recommend that travellers from Serbia should identify the reason for their return before they travel to the UK. They should ensure that they take evidence both for the need to travel to the UK and for returning back. We do not know what evidence would prove sufficient but recommend that everyone should carry evidence of Serbian residency (or identity card).
Once again, whilst we have made every effort to summarise this advice, we do not accept any responsibility for anyone relying on this summary; and we recommend that they look at the regulations and guidance to ensure that they are complying. The regulations can change at any time without notice.