Get an old bike box from a shop they're always throwing them out ,fit bike in best you can [well padded]seal and transport to the LARGEST po in ur area
,if its all 'parceled up they'll be happy to take it
the big green parcel machine are good so is dhl ,parcelforce are ok but if you google the words parcelFARCE there quite a lot of horror stories,Any carrier that has tracking is usually fine[u get what you pay for]
If you dont have a car to take it to the po on,ask them to pick it up at ur home address,failing that ,a skateboard is good to transport it on