The UK has not said people must leave (US has). The language is that people should review their need to be in Ethiopia which is one step back from saying leave. The advice is however against all travel to anywhere other than Addis. A slightly pedantic difference perhaps, but an important one if you are there for business. The UK government has temporarily removed the requirement for anyone to take a PCR test before flying to the UK which makes it much easier to leave at short notice.