Looking for a standard oval toilet seat with a not so standard hinge?  Take a look at the RTS 240 mm Wide Hinge Oval Toilet Seat.  It's a Duroplast seat with slow closing seat and lid:

https://replacementtoiletseats.co.uk/ourshop/prod_7281853-RTS-240-mm-Wide-Hinge-Soft-Close-Toilet-Seat-w-One-Button-Release.html

These seats are sold with adjustable hinges to accommodate a comfortable 240mm width when measuring from the centres of the pan holes.

240mm Wide Hinge - Duroplast - Oval - Slow Close - Adj Hinges - 1 Button Quick Release - Sorted!