The Galaxy S9 is a fine phone, but it just launched and its price hasn’t had time to cool off. The Galaxy S8 is a different story. It’s price in the UK is under 500, compared to 740 for the S9.
You can find a new unit from Amazon for 475 or from Alternate for 490. It’s the G950F model with Exynos 8895 (single SIM).
Samsung Galaxy S8 G950F
You can go lower than that by getting a seller-refurbished unit from eBay. A Vodafone unit in Pristine condition is 430, the unlocked one is 470, so not really worth it. Note that this is for the Black model, other colors cost more. Dropping down to Good condition, the lowest price is 360 (Grey phone for EE), which is at least 100 off a new unit.