If you order before 2pm Monday-Friday, you can choose next day delivery, yay! You'll then receive a text from the courier on the day of delivery confirming your one hour time slot. This time slot can't be changed, but if you're not going to be in, then you can ask the courier to leave your parcel in a safe place, or deliver to a neighbour!
If you need your order the next day and as early as possible, then there are options for pre 12pm and pre 10:30am delivery in most areas.
If you choose a Royal Mail delivery option, then you can't choose a specific day, but you can opt for tracked delivery so you can check where your parcel is on its journey!