I'll share my recent experience chasing an R5 rated sovereign, the 1888 1st legend jubilee head from the London mint (DISH L8).
It's rated as R5 in Marsh and Steve Hill thinks it should be upgraded to R6 (3 or 4 known to exist). Here's what he writes:
'This is an extremely rare coin, there have been no examples present in any of the major auctions, including the "Bentley Collection" sold by Baldwin. It is far rarer than the current valuation suggests in the "Spink Standard Catalogue" and since the DISH publication was released indicating the rarity as R5, it is now thought that it would be more accurate to call this variety Rarity 6.'
https://www.sovr.co.uk/products/victoria-1888-sovereign-london-first-legend-extremely-rare-variety-ei16305
John Blyth had one for sale recently and says he has only ever seen two others for sale.
https://jonblyth.com/coins/coin-details.aspx?id=2450
And another Sovereign rarities, sold for £1500:
https://www.vcoins.com/en/stores/sovereign_rarities/263/product/victoria_gold_sovereign_1888_l8_r6/968584/Default.aspx
The PCGS population report shows 3 coins in the database: https://www.pcgs.com/valueview/victoria-1838-1901/1888-sov-s-3866-1st-obverse/4015?sn=207059&h=pop
However, I have seen three of these coins (not slabbed) pass through UK auctions only this year:
For £1200, at London coins: https://www.londoncoins.co.uk/?page=Pastresults&auc=176&searchlot=1989&searchtype=2
For £750, at the Coin Cabinet: https://auctions.thecoincabinet.com/lots/view/4-5NX9XR/1888-gold-sovereign-first-legend-very-fine-agw02355-oz
And £855, on eBay: https://www.ebay.co.uk/itm/125611773560
So with the 3 for sale/sold by the dealers, 3 in the PCGS population and 3 sold at auction this year there are potentially 9 out there. And I guess there will be more to surface that haven't yet been recognised, so I'd guess this coin is heading towards R4 rather than R6. I've not seen any in really good condition, most are about fine to very fine grade, and prices range from £750 to £1500.
Best of luck with the hunt!