This depends on which courier you are wanting to choose or you‘ve already booked with. Each courier has their own drop off locations in the UK so please make sure that you go to the correct associated with the courier.
Here is a list of drop off location name for each courier.
Parcelforce: Parcelforce Depot or any Post Offices in the UK
DHL: DHL Service Points
UPS: UPS Access Points
DPD: DPD Pickup Shop
Find your nearest Drop off point here.