Depends if have a lot of money and want to be assessed faster,or can wait long but get it for free?
NHS assessment-start at the GP,bring a list of all the symptoms that do have,and why want to be assessed,they will refer onto a pyschologist or pyschiatrist,and ask if they can specialise in ASD if possible.
Private assessment-start at the National Autistic Society- http://www.nas.org.uk contact them and they will give a list of private pyschs available.
[Edit]
^have said mostly what Kaleido has said,took that long to post.
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>severely autistic.
>>the residential autist;
http://theresidentialautist.blogspot.co.uk
blogging from the view of an ex institutionalised autism/ID activist now in community care.
>>>help to keep bullying off our community,report it!