@Phineyj yes, NHS paediatric dietician. (We've been before to a different NHS dietician with my other ARFID daughter (DD1) and were significantly less successful that time; but with the DD in question (DD2) we've really lucked out with who we ended up with.)
She also talked a lot about interoception with DD2 again, DD1 is clearly unable to process / understand her own body signals, but it was a useful conversation to have with DD2 who actually has the opposite problem she is hyper aware of everything going on inside her.
As @Airelles suggests might be the case, CAMHS / ED service said they couldn't help us because DD's eating challenges are not "text book" and "she has ASD so it's complicated and we can't help" (not exact words, but that was the drift). It felt very isolating: the message I got was yes, this is complicated and outside anything we have a manual for, so no-one can help and you are on your own -- which is why I wanted to respond to OP. This can feel like you are on your own and it's not clear where to go / what to do next.
@Airelles suggestion of allowing your DS to choose what professionals he works with has also been key for us: I had been suggesting therapy for DD2 for some time, but the one we have ended up working with (££££ eek) was suggested by someone whose advice she really trusts (and who knows someone who worked with her). When I suggested therapy I was met with a brick wall (what does mum know, right?).
Also agree with @Tickingcrocodile DD2 (and DD1 to some extent) are much more likely to do what a healthcare professional says if it is made very explicit (and is something they feel that they can achieve the eat three meals a day plus lots of red meat and green vegetables advice does not land well). It has been the thing that turned it around for DD2 -- she is now working with two professionals she really likes and who she feels get her and she is (to some extent) willing to listen to them and try what they suggest.
And yes yes to A&E if calorie intake doesn't go up.
Please feel you can keep posting here for advice, to share what is happening, or just to moan. The worst thing for me was the isolation. Good luck to both of you x