Some schools are a bit pickier about dress codes than others - for an interview, I'd still suggest smart dress (with tights), smart shoes and if you have one, a jacket or at least a new cardigan. They expect men to turn up wearing suits, so it's only fair - even though some women will turn up on the day wearing leggings and trainers, it doesn't fill staff or kids with confidence (male invigilators usually wear smart trousers and shirt/jacket without a tie). Not clip-cloppy shoes, though. Never clip-cloppy or squeaky shoes.
No barking commands at kids. They really don't like it at the best of times, least of all when they are stressed.
There will be kids who have additional access arrangements, such as extra time, taking the exam in a different room, having somebody act as a reader or scribe, using a laptop (which is then subject to absolute rules on it not getting left unattended). For listening exams, you might need to play an audio track, so would need to know how to use a music player such as VLC or operate a CD player/play a CD in a computer connected to a PA system.
Take your ID documents in case they want to do a DBS check straight away - passport, driving licence, council tax bill, water bill, that kind of thing.
You'll be working fairly independently/in small numbers - teachers aren't allowed in the halls (and they do try frequently) or to get the attention of kids - so they want to know you're comfortable being responsible/adapting to changing situations.
They might (if they're feeling particularly mean) ask you things like what you would do if you discover somebody cheating (hint: you don't yell and drag them out of their seat) or they are late and try to get into the hall after the exam has started. But largely, they want somebody who is happy to do any type of exam and can be calm, quiet and authoritative without ever needing to raise their voice.
Training isn't very common. You'll probably get a huge bundle of papers to read beforehand, but you'll learn on the job, most likely paired with a Senior Invigilator.
It's an important job, it has to be done right - but if you can come across as calm and unflappable - and capable of remaining silent for 3 hours without tapping, fidgeting and generally being annoying to stressed candidates, you'll nail it.
Good luck!