Speaking as someone who has run a review site for rather a long time, I thought it would be a good idea to add a key piece of information...
When something costs under £30, we generally expect whoever sends it not to ask for it back. In fact when it comes to DVDs and Blu-rays, when studios have been so "cheap" as to send us a screener and return it when we've finished, we often just return it straight away unreviewed. It's like, we are going to spend hours of our time on this, and you can't even foot that small bill of £5 or whatever it cost you to make that screener?
Once things get to the £40-80, we keep quiet about them but if the company that sent them asks for them to be returned after we've finished, we don't begrudge it too much, as long as we aren't paying for it. They'll arrange the courier and just ask us when we can be in for them to collect it.
Everything over that, nearly always gets asked for back! Which is fair enough.
I guess what I am basically saying, is you should be able to get a fair amount of coverage with just a small handful of loaner units. Just send them out to the people with the biggest readership first, making sure they agreed to review the "loan unit" before you send it, and offer to answer any questions and give any help they might need, then do so ASAP because there is nothing more annoying than asking a key question and having to wait a week for an answer when you are trying to review something.
The moment they put the review up, thank them very much for their time, regardless of how lukewarm or bad it is, because even a badly researched and written review takes time, then arrange to have a courier pick it up from them and either move it on to someone else or (probably better) return it to you so you can check its fine and repackage it all up with any bits that went missing.
If you do get an unfair review, don't get confrontational about it, take anything on board which is fair and just accept it, anything which is there as a result of inaccuracy you can diplomatically point out as unfair.
Good luck!