if you did get an agent you would have to do exactly the same thing for the agent as you would applying for the evisa yourself. Sorry I don't know of any agents to help you in the UK . I am not very computer savvy but I did the application in about 15 minutes just using my phone it is an extremely easy process
If you arrive it will be 30 days visa exempt very easily extended . I have never been asked for onward flight at immigration but your airline might ask you for it before allowing you to board the plane. It would be best to check with your airline company to check their requirements.