Have used gateway2lease, vehiclesavers and Vertu in the past – all were fine. I don’t have any loyalty to any of them, it’s just been who happened to be best for a particular car at a particular time. You’re typically just dealing with a broker who’s sat between you and the actual finance house or lease company. All they really need to do is fill out some paperwork and make sure the car arrives.
Look at leasing.com and What Car? leasing sites to get a feel for what’s decent value, then you can ask for a proper quote from the broker (if you need different mileage, etc). I stick it all in a spreadsheet just to keep track of things, stuff like admin fees can vary a lot and make a cheap-looking deal less so.