The key is to find somewhere trustworthy.
This place is particularly good as they double up as a Nikon and Canon Authorised Service Centre, therefore all their kit is always in good nick. As far as I'm aware they've got a pretty good reputation all round amongst the pro community.
http://www.fixationuk.com/
I have no association with them apart from having used both their repair and rental services at various points in the past.
Would obviously be wise to ensure your own personal insurance covers any liabilities you may incur whilst having a rental unit in your possession. No different to renting a car really.