Although its no longer the day job, I'm still involved in the motor trade in Paisley and I have never heard of a waxoil specialist, I assume it must be a franchise of some sort.
I'm and a panel beater to trade and I used to waxoil vehicles regularly, and as others have previously said, I would get the underside jet washed thoroughly then inspect for corrosion as I regularly found holes and large areas of corrosion that required repair.
Folk used to think that the £150 to wash and waxoil the underside of a car included the 2 weeks of welding as well
There were also those who think waxoil has Christ like properties that turns rust to beautiful metal again