If you really want her to do the exam, you can register as an entrant on the ABRSM website and do it all online, as long as you register at least a few days before the closing date - the next one is fairly soon, so you need to get on with it if you are going ahead.
She will need to go to your local exam centre - usually the nearest big town - you choose the centre at the time of the entry. Be aware that these can be a variety of venues, often someone's house, and the quality of the pianos is variable to say the least - not necessarily a problem at this early stage. The Prep Test is a nice gentle introduction to exams and they come out with a certificate on the day.
There's no reason why your DD couldn't carry on working towards Grade 1 and do the Prep Test as a warm-up. She could even use one of her new pieces as the own choice piece for Prep Test.
I would also suggest moving onto grade one rather than paying for a prep test and the accompanist/ depending on the set up at the centre. She has already learnt the pieces, perhaps her teacher or yourself could print a certificate to give her and ditto to playing pieces in concert or school assembly. If she is going to do all the grades it will become expensive so saving a bit at this stage is good.