i have sent a FG Leopard, had my job lot of Baja 5b collected from Scotland and then the 5b sent out to London and 2 Marders collected from Wales, i find the best place is http://www.interparcel.com/, click quote and book and enter the details they will give you a list of companies and their prices and then collect the parcel at a selected time and date from you.
its normally below £20 but if you have a printer can be below £10
also i have found that every time i get a parcel collected, by whatever service, it arrives next day might just be luck though
oh, and about the box size problem, i normally make my own out of 2-3 boxes