As a signature is required upon delivery, if you are not home when Royal Mail attempt to deliver this, they will take this to your local delivery office where you can collect it with some form of ID. Don't worry, they will leave a 'Sorry We Missed You' card explaining your options!
If your order is over 4kg, this will be dispatched with DPD instead. If you are not home when DPD try and deliver this to you, they will keep hold of the parcel and deliver this to you the following day (Mon-Fri). DPD will be in touch and may allow you to request a safe place for your order to be left, if you know you are not going to be home when they try to deliver this to you.