You should post your sample in a Royal Mail postbox. You can find your nearest one here.
You may see a parcel collect option printed on your return envelope Royal Mail label. Our laboratory also provides covid test kits with return Royal Mail envelopes and so if you provide the label tracking number, you may see a message saying that Royal Mail are unable to collect your parcel because they won't collect covid test kits. This is because once shipping labels are printed, Royal Mail cannot distinguish which labels have been used in which laboratory test kits.