The information in this article is correct at the time of writing, but please do check with the company in case their terms and conditions have changed.
This article explains one way to ship goods from the United Kingdom to Japan. If you have bought some heavy items in the United Kingdom and don't want to carry them onto the aeroplane with you, you can arrange to have them shipped from the UK to Japan via "Kuroneko Yamato".
They ship excess-baggage style items from the UK to Japan up to a maximum of twenty-five kilograms weight, either by sea mail, which takes several months, or by air, which takes a few days. The sea mail option is slightly cheaper.
They can collect items from your house or you can take the items to their office in Piccadilly or their depot in Harrow. They offer a small discount if you take the items yourself. Pickups are available in London for no extra charge or outside London for an extra charge depending on where you are. You cannot specify the time of collection.
Their office is located inside the Mitsukoshi shop in Piccadilly in London. It is just a short walk from Piccadilly underground station. The staff speak English.
The cost of the shipping by Kuroneko Yamato is very reasonable compared to other shipping companies.
There is no information about the service on the web, but please see Kuroneko Yamato for the address and telephone number of the London office. This page is partly written in Japanese, but the address and telephone number are presented in English.