I have found this deal by accident yesterday, so thought I will share it with you as this is the best price for the new Pixel 5 device I can currently find (pay as you go).
1. Go to the website.
2. Click on "Pay As You Go available for £599
+ min. £10.00 Add-on"
3. Price should adjust in the basket.
4. Remember to change "Pay As You Go Add-ons" to "£10" one.