Remember also that you're seeing the price with VAT added. The pre VAT price is 2500. Still expensive, but HM Government is still taking 20%, or 500, off you.
Other than that, yes, law of diminishing returns on price vs sound quality and yes on decent cabinets rather than MDF or God forbid injection moulded plastic and yes on as your ears get more accustomed to high end hifi the more you'll want it. Same idea really as buying a cheap hatchback vs a sports car. Both will get you to work and back and to a greater or lesser extent bring the groceries home, but the sports car should be more fun when the going gets twisty.
I see the main difference, apart from the number of drivers and not having heard either, with the A7s vs, say similarly priced ProAc models (Studio 148s?) being that the Spendors are designed for single wiring whereas the ProAcs are designed for bi-wiring. You pays your money, you makes your choice.