IMHO it is a risk handing over your car at an airport to someone I don't know. Would I do the same at my local shopping centre? No, so I wouldn't do it at an airport.
But I have no knowledge of this company, they don't say where the car will be parked. Looking at Google Earth Gatwick Road is an industrial road and difficult to say if there is a secure parking lot. They say it's council registered maybe you could check, their web site is pretty basic. Are the drivers insured?
There have been many reports of cars being parked in streets and drivers not having insurance.
For me, I always use a car park listed on the official airport web site or I use Holiday Extras. I prefer to park my car so that I know where it is and that it's safe. Just my opinion, only you can decide if it's worth the risk.