Selling computers in various parts of the world is subject to wildly different business conditions in each sales entity.
It is subject to exchange rates, but also the implied warranties required and the peripheral costs of the entire business enterprise in each area, including the parts and the people required to support the product and repair it. Also, the UK does not have quite the economy of scale of the US market.
US list prices do NOT include State taxes, non refundable costs that vary from typical five percent to as high as 16 percent additional in New York City, for example. UK prices tend to include VAT, which may be comparably high and are typically INCLUDED in the posted prices, as I understand it.
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When you start by specifying a MacBook Pro, you pay dearly for the stunning built-in display and the battery operation and portability. If you are installing 128 GB of RAM, it seems like you have other tasks in mind than web surfing while at the shore.
A Mac Studio with 128 GB RAM can be had in the US for nearly US$1500 less. (not portable, no display)
Readers here are NOT Apple employees, so we can blurt out: if you don't want to pay that much, you are welcome to buy whatever system meets your needs in any fashion, including systems from different makers.