The methods you speak of cost more (in labor) for EVGA. EVGA honoring their pre-Brexit policy and not moving mountains is well within their rights.
It is your country's fault that Brexit happened, not EVGA's. EVGA did what they were supposed to—replace your GPU. You choose to stay in England, as an immigrant, meaning that while you may not like Brexit, you consent to the country's laws.
I'd love to hear how typing in the correct tariff code and adding the original invoice to the customs paperwork costs them more money in labour. Filling out customs paperwork correctly isn't "moving mountains", it's the basic order of doing business. If EVGA doesn't want to do this, then they should stop supporting their UK customers full stop.
Also this isn't a "pre Brexit policy" - if you buy anything from EVGA right now here in UK, are you aware that having it repaired/replaced will incur customs duties due to EVGA's incompetence? You won't have this problem buying any product from almost any other manufacturer, so not disclosing this information is dishonest at best and fraudulent at worst.
To do discovery to find the proper forms, and what they have to fill out in them, and to get their legal department to make sure they're doing it correctly, has significant costs.
You bought your card before Brexit. They're honoring the deal they made with you—not their fault they had the ground pulled out from under them with laws you consented to.
It is your country's fault that Brexit happened, not EVGA's. EVGA did what they were supposed to—replace your GPU. You choose to stay in England, as an immigrant, meaning that while you may not like Brexit, you consent to the country's laws.