The first thing for me to do after arriving at London was to utilize the mobile telecommunication and the internet. NTTDoCoMo and other carriers in Japan also provides international call and mobile networking without any stress. However, it is quite expensive compared to domestic use. Therefore, it would be better to make a contract with a local provider if you stay there over one month.
Fortunately, it was so easy for me to solve this problem. My host offered me a SIM card powered by Giffgaff for free.
Giffgaff is a Mobile Virtual Network Operator (MVNO) using a certain UK network. It offers several plans, including telecommunication and networking. The minimum cost is only 5 pounds per month.
I applied to a course with phone calling and unlimited internet browsing. Unfortunately, tethering is not permitted. It costs me 12.5 pounds per month. Calling for Giffgaff users is free. A total of 250 minutes domestic calling fee is included in my plan. Amazingly, international calling is not so expensive. Calling to Japan takes only 4 (landline) or 12 (mobile) pence per minute. Furthermore, the extra 5 pounds are attached with the contract.
Contract procedure was quite simple. I input the serial number written on the sheet via the internet. APN setting was needed on the smartphone after inserting the SIM card. You have to remember that the SIM locking should be dispelled in advance if you use a smartphone which is bound to a particular provider.
The connection speed was a little bit slow. Nonetheless, I am satisfied with it.