Airline network are a pretty long established UK based discount flight shop - so they'd be pretty reputable and ABTA bonded etc. so nothing to worry about there.
I'm sure it's fine and will book to one of major hire companies anyway, you'll just pay/part-pay Airline Network and then get a voucher that you give to the car hire company for your rental - you'll be actually renting the car from Hertz/Avis etc., just you get a better rate going via Airline Network than going direct.