I posted mine to Penzance and back using Parcel2Go! I think it cost me under £10 each way at the time (about 4 years ago ). My ex's sister lives there so I sent it to her. For the return leg, I got the courier to pick it up the morning I was coming home so I was back up here by the time it arrived.
I blagged a cardboard bike box from my LBS but it turned out to exceed the courier's size limit so I cut it down to size and managed to squeeze my bike in by taking the wheels, saddle and bars off.
It might be worth considering it if you have someone who would sign for it and hold it for you - perhaps you could offer a bike shop £10 or £20 to do that for you? Carriage both ways plus the backhander would still work out cheaper than renting and you'd be able to ride your own bike.
(Make sure that your bike is covered.for loss or damage in transit!)
