However, NICE (the National Institute for Clinical Excellence) has published guidelines about diagnosis and the short version iirc is that you're well within your rights to point out that your GP is not a specialist in this area and ask to be referred.
The bad news is there may not be anywhere locally to refer you to, so you may need to travel out of area, pay to see someone private (and then fight to have your prescription accepted and paid for by the NHS). There are more kids' specialists but they may not recognise the nuances of the adult condition.
No, I haven't done it - I've been meaning to for over a year and keep putting it off, ha ha.
Links: https://www.nice.org.uk/guidance/CG72 - NICE guidelines; https://aadduk.org/forum/ - AADD UK forum