”Parcel Monkey” are probably your cheapest/best option. I’ve sent a whole bike via them before for about £8 plus insurance and that’s next day too. As long as you stay under the (pretty generous) maximum length & weight it’s extremely good value (standard bike box is within the max.
Very easy to organise too, as you can order it all online the night before (even pretty late) print out the label, stick it on and have it picked up the next day.
With insurance etc, it can add up (I just sent most of a bike with insurance up to £900 and it was £20), but it stil seems very good value, so I’m very happy to recommend them.
Despite the name I have no connection with the company other than being a very satisfied customer! 😀