ivykaty, although the theory is that social services will sort accommodation, the reality for 16/17 year-olds is unfortunately not at all good.
Firstly, everyone's efforts will be focussed on getting the parent(s) to allow the yp back into the family home. This will almost certainly include 'professionals' telling you there is no accommodation available, that you have no choice, that you have parental responsibility, that you are a bad parent...
Next, the young person involved has to ask for, and consent to, any accommodation... If parents throw them out, they have to iunderstand that they then have no say, control or choice about where they go... For example, my own son refused point blank to go to the housing office, and preferred to sleep on the sofas of exactly the people I wanted to keep him away from...
Generally, bedsits/flats are ONLY offered to under 18s who are already in local authority care, because under 18s cannot sign tenancy agreements. The LA will sign for a young person who is a 'care leaver', but does not do that for 16/17 yos who are 'newly homeless'.
There are a very few specialist supported accommodation units for 16/17yos in a few areas... Again, these are most likely to be offered to 'looked after chaildren' who are not yet ready for fully independent living.
If accommodation is offered, this will generally be in a B&B/hostel. Even high priority young people (e.g. my friend's son who has Aspergers) can spend several months in this situation. Obviously it's not very nice.
Very significantly, accommodation for teenagers is in such short supply that (in most areas) if social services are really concerned that a teen is a serious threat to the safety of a younger sibling, they are more likely to take the younger child/children into care and leave the teen in the family home.
So all in all, I'd say social services were a non-starter under these circumstances :(
I remember a few months ago someone posted a link to a residential school run by a charity... It was free I think... Focussed on developing a trade/skill... I've had a good look back through previous posts for details, but I can't find the post... Perhaps someone else can remember more...?