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I sent them a low offer of 1500
I don't think that counts as a low offer, if they're going to let it go in that way they'll bite your hand off. Depends on their policy, I do a lot of IT returns from people WFH and the companies are paying more for transport than the kit is worth because they have to sanitise it for security, data protection and commercial confidentiality purposes, and keep control of the disposal path for CSR reasons. Obviously if it's already in their office and you're offering to take it away it simplifies the logistics, but you're still possibly asking them to divert from policy.
Polygon
gbj_tester
Quick internet suggests depreciation of roughly
which translates to £1,645 current value.
PCpartpicker reckons more than that for just the CPU+GPU+RAM, so I'd try for the depreciation angle.