I'm not considering it at all. I'm shocked they are even selling it.
I wouldn't look to the Apple Refurb Store for second-hand bargains on old models - Apple have never recognised the idea old designs depreciating in value. If, however, you're buying a
current model that's maybe rumoured to be upgraded soon, it's worth checking out as a way of saving a few quid on something that is indistinguishable from brand new. The prices of the 2019 models
As others have said, that "2013" Mac Pro is only "discounted" in the sense that it probably cost £6000 new - but it might be attractive to someone who
specifically needs that model and is prepared to get it in "as new" condition rather than take their chances with a third party seller.
The current exchange rate is £1 = $1.22
but most published UK prices already include 20% VAT (sales tax-ish). So, for a ball-park estimate, divide the £ by 1.2, to get rid of the tax, then multiply by $1.22... so
very roughly, $1=£1 isn't too far off when comparing US and UK prices.
Strangely, the $1=£1 rule for sticker prices of personal computer products from US-based firms has been prevalent since the late 1970s, even through times when £1 was closer to $2 and VAT was not only lower but wasn't included in advertised prices for computer gear. These days, it's more defensible.