It’s not random. 
Brenna picks the testers best suited to an item, and this can be for a variety of reasons (eg. they want a couple’s perspective, or a newbie, or someone who loves butt plugs, etc).
The main bit is that you need to have live reviews already up on the Lovehoney site, so they can see what your writing style is like. If you’ve done lots of reviews but they’re just short “This was great!/This was shit!” then that’s not going to move the dial a lot. You need a handful of well-rounded, reasonably thorough ones to prove that you can do it.
The other thing is to send your measurements in for the lingerie testers. That improves your chances a lot for those ones. 
(I’ll chuck a link in for the The Great Big Review and Testing Guide too, in case you hadn’t spotted it)