Just a few ideas - off the top of my head, and not knowing how old your DD is or how severe her autism is. Or if it is long enough flt to have to sleep. Partly based on our experiences with DD (Aspergers, ADHD) and other advice I've seen around since.
Rescue remedy is good for calm - there are a few different types including at least 1 specifically aimed at kids (daydream). We had a bottle of dozol with us also on long haul in case we needed it (we'd been warned to try it on DD beforehand, as it makes lots sleepy but can make some kids wired instead). And also teetha, as we still use that (she won't drink chamomile tea but quite likes the granules).
We went before we were even on the assessment road (we'd asked, but hadn't started it) so I was going on general "travelling with kids long haul" tips and previous experience of DH and my LH travel, and our somewhat limited recognition of DD's limits.
We had gone short haul to UK from Ire when DD was 4.5. The problem with that one was change in routine - she thought we were driving to Granny's house for the weekend, when we'd planned a surprise flt to London for weekend. Once she got over "not going to Granny's house", and into the car for airport, she got all excited and enjoyed it. I talked all the way there about what we'd see and expect, and do in London.
Based on the "no surprises" tactic, when we booked a transatlantic hol (DD 5.5), we told her all about it beforehand. Explained all about getting to airport, checking in, passport checks, security and taking off shoes maybe, getting something to eat in the airport (early flt so tiny snack at home and planned starbucks we knew was in apt), much bigger plane, long trip, lunch on plane, another smaller plane, remember how toilets are quite different on planes and make strange noises.....the whole 9 yards.
Manchester Airport has a good guide to do that beforehand, has lots of pictures and activities to do as well.
I hear lots of UK apts actually have facilities to help manage ASD people - help at checkin and security, someone to accompany you who knows airport etc. Could be worth looking at.
For long haul, we were renting a car so got a trunki bakcpack booster seat (same price as hiring booster on arrival, and have it forever now). DD was able to carry that herself as her carryon and pack it herself with the toys and books she wanted on the plane and hols. She also carried her cushion - a cushion she's had for years that regularly gets slept on or snuggled, and is very bright so hard to miss!!
In DH's carry-on, I had change of clothes for us all (always on LH anyway) and dressed DD in comfy clothes (she lives in tracksuits anyway). And layers.
In my carry-on, I had things for flt. Things like a new comic (she chose but not opened until plane), reading book for story, colouring pages and pencils (standard item in my handbag for coffee shops etc anyway), a teddy she loves snuggling (one of the small ones), snacks etc. And I had also got a few small things to stave off boredom - couple of new, cheap toys and another new comic, all wrapped in paper, which were allowed out on a "1 per hour" basis if needed.
We also had the iPod nano with some new movies and favourite music loaded, headphones, and a DS game. I had a book each for me and DH, but not really expecting to read them. And there are the in-flt entertainments and meals as well.
Coming home, we DID go overnight. So I had told DD all about that too. And we had agreed that she would change into her pjs at the layover point (short internal flt then on from JFK in early evening). So we changed in airport (easiest), and she came out wearing pjs, crocs and slipper socks, carrying her backpack and her cushion to board (lots of cute "aw"s from other pax around the place, she was proud as punch strutting around).
Got on, she ate, and then settled down to "sleep". She found it hard to GET to sleep (it was VERY full flt, incl sports team who didn't want to sleep, and I was wary of using dozol as hadn't tried it before so just teetha), but relaxed and did stay in seat. Eventually both her and DH got a few hours sleep tho.
The talking point since has been how they slept so long they missed brekkie and DH got nothing at all!! (she got a banana (grudgingly) when we asked AH, also they v grudgingly let her go to toilet as "coming in to land" even tho signs not on yet and still half hour out - just didn't want to move trollies blocking [grr]).
She was absolutely FINE. She had known what would happen, and when, and what it would look and sound like (warned of crowds and strange noises, sometimes high pitches can hurt her ears). She had ownership of her things in backpack. She was dressed appropriately for sleep (signal to herself).
It was a lot of prep, but went fine at the time. And I was ready to take her to a quiet(ish) corner if need be. (Just having the backups in your mind can help you too). And allowed plenty of time for each step and things to distract from boredom (both well loved and new).