Just checked the shipping costs. Up to 15 kilograms the shipping cost to Great Britain is 46£ and up to 30 kilograms it`s 74£. I think the bike and a proper package would weigh something between 15-20 kg.
There are some restrictions also; the lenght of the package must be under 150cm AND the lenght + circumference must be under 300cm. This means some disassembling like taking the wheels, cranks, pedals etc. apart. I think 120 x 80 x 50 cm box with lots of cushioning would do the job just fine.