Please see gov.uk/guidance/tier-4-stay-at-home for full details
If you are in Tier 4, you should not be travelling abroad unless it is permitted. In addition, you should consider the public health advice in the country you are visiting.
If you live outside a tier 4 area you may still transit into or through a tier 4 area to travel abroad if you need to, but you should carefully consider whether you need to do so. In addition, you should follow the public health advice in the country you’re visiting.
If you do need to travel overseas from a tier 4 area (and are legally permitted to do so, for example, because it is for work), even if you are returning to a place you've visited before, you should look at the rules in place at your destination and the Foreign, Commonwealth and Development Office (FCDO) travel advice.
UK residents currently abroad do not need to return home immediately. However, you should check with your airline or travel operator on arrangements for returning.