If you trust that you've made the right choice with the hair and makeup artist, ask them when it's a good time to do the practice run. In most cases it would be about 3-4 weeks before the wedding. If you have doubts and think that the practice run should be more of an audition then I recommend that you schedule in the Trial as soon as possible so there is enough time to find someone to your liking.
The average cost for a trial run is about £150 and the H&M artist should dedicate at least 3 hours to you on that day.
Make sure to research images of the look you're after and have pictures of your wedding dress and any other accessories you'll be wearing to show at the practice run.
Even though you will be armed with loads of pictures you've been collecting, always ask the professional their opinion, they can look at your face shape and hair type and make some valid suggestions. Be ready to try a few different looks and in some cases ending up with something you love that you never thought you'd go for.
Most important thing to remember is to have realistic expectations of how you'll look.