This is your best bet and overclock it:
Team Group Xtreem "8Pack Edition" 16GB (2x8GB) DDR4 PC4-32000C18 4000MHz Dual Channel Kit - Black @ £209.99 inc VAT
TXBD416G4000HC18GDC01, 4000MHz RAM Speed, CAS 18-19-19-39 Timings, 1.30-1.40v VDIMM, Lifetime Warranty with OcUK.
Only £209.99 inc VAT.
ORDER NOW
Or go mental with this stuff:
Team Group Xtreem "8Pack Edition" 16GB (2x8GB) DDR4 PC4-36000C18 4500MHz Dual Channel Kit - Black @ £289.99 inc VAT
4500MHz RAM Speed, CAS 18-20-20-44 Timings, 1.45v VDIMM, Lifetime Warranty with OcUK.
Only £289.99 inc VAT.
ORDER NOW
But you need absolutely the best mainboard (Asus) and a cream of the crop CPU to hit such frequencies, though 4266MHz would be no issue for such a kit as long as CPU and mainboard are upto scratch.