Homelessness is when you have nowhere that you can live. Being homeless and being a runaway are different things – if you are have left the place you were staying and are choosing to not return rather than being unable to return, then you are a runaway and not technically homeless, whereas if you have nowhere to go and have no choice but to be on the streets, this is being homeless.
If you are 16-17 and become homeless, you are more likely be able to get accommodation through the council or emergency housing – if the council thinks you are in “priority need” then they might be able to find you somewhere to stay while they look for a more permanent place.
If you have problems with this then contact Shelter, who should be able to help you. If you are aged 16-25 then you could have access to foyers if you are in employment or education – foyers are set up to help prepare you for independent living and can offer you some support while you are there. You usually have access to foyers from 6 months to 2 years. If you are younger than 16, then you cannot live independently and social services should sort out a place for you to stay with an adult to care for you like foster care or a children’s home.