I have never used a courier for anything before so dont know the form. If insurance is extra I would recommend taking it. Bike will be protected well. Rear derailleur is off and tucked inside the frame .Forks will be protected and I will mark the box FRAGILE.
Im sure that they rarely damage anything but shoot happens.
Just been back to
Evans. They dont have bigger boxes but showed me how to pack it and it goes in easy. Theres foam lagging around the tubes and bits of polystyrene and cardboard jammed around it plus old towels and stuff in the gaps.Rear wheel is still fitted, derailleur is removed and tucked into the frame. Bars are off Stem turned round Pedals and seat/post off and packed in.A fork protector is fitted with the front wheel skewer.
I cant do any more now its as protected as possible.
With all the gear and brake cables diconnected its going to take several hours to reassemble and set everything up again so dont be impatient Ed (I know I would be) get someone to help you and plan on it taking all day. Use a bit of grease on the seatpost and any threads you put back together.
The gear cables are almost a year old so would be a good time to change them now .I have taped a pair of new ones inside the box.
Take your time, be methodical ,slowly/carefully is usually quicker on jobs like this.Good Luck