A quote that is markedly cheaper than every other operator is worth questioning rather than celebrating. Airport transfer pricing does not vary wildly between legitimate licensed firms, because the underlying costs — licensed driver, insured vehicle, fuel, parking, the return leg — are broadly the same for everyone. A price well below the market usually means something is being left out, most often waiting time, parking or the meet-and-greet you assumed was included.
Other signals are simpler. No written confirmation, no company address, a mobile number as the only contact, cash-only payment, or vagueness about licensing are all reasons to look elsewhere. So is an operator who will not confirm what happens if your flight is delayed — because at Luton, sooner or later, one will be.