Extend a temporary visitor visa?

I visit Japan every year as my partner lives there. I am from the United Kingdom.
My boyfriend lives in Japan and I am hoping to stay there for 6 months next year on a temporary visitor visa I will not be working and I will be using my time go sightseeing, usually on entry you are granted a 3 month visa but you can apply to extend this. I will return to England at the end of my trip after 6 months.
I was just wondering if anyone has had a similar experience to mine and can let me know what applying for an extension on a temporary visitor visa is like and would I be likely to be granted the extension.

Applications are decided on a case-by-case basis so unfortunately nobody here can say how likely or unlikely it is that you will be successful. Technically it is allowed, if anyone tells you anything more than that it's nothing but conjecture/speculation, unless they are an immigration officer. Try to get an extension, or seek out an alternative instead. Visa runs to Korea, Taiwan, etc. are cheap and easy, so that might also be worth considering. Good luck.

British citizens and citizens from 6 other countries are allowed to get a visa extension of 3 months without any problem.

Take a look at the Ministry of Foreign Affairs of Japan website page on visas:
https://www.mofa.go.jp/j_info/visit/visa/short/novisa.html

(Note 7) For nationals of those countries with visa exemptions permitting stays of up to 6 months under the bilateral visa exemption arrangements, those who wish to stay in Japan for more than 90 days are required to apply for an extension of the period of stay to the Ministry of Justice (Regional Immigration Bureau) before the period of permitted stay is to expire.

without problem is incorrect.
Still there need a valid reason for this extension.
The application which has to be filled in mention this reason and all documents need to be handed of to the immigration. The immigration officer is deciding if the reason is a valid reason for extension or not

Yet, bilateral visa exemption arrangements are still bilateral visa exemption arrangements and citizens of those 7 countries have better chances to stay 6 months in Japan than citizens from the USA for example. I never heard about extensions being denied for UK citizens with valid reasons like wanting to visit all Japanese prefectures or travelling on bicycle from north to south.

OP should just avoid to speak about the boyfriend living in Japan and use the "tourism" as the reason for extension and it should be OK. Plus she's a woman...sometimes it helps.

Thank you all for your responses, I feel more positive about applying for the 6 months now.
How early can I apply for the extension for example can I enter Japan on my 3 month visa and the text day apply for an extension or will this need to be done at the end of my initial 3 months?

How early can I apply for the extension for example can I enter Japan on my 3 month visa and the text day apply for an extension or will this need to be done at the end of my initial 3 months?

It says right in T.K.'s post...those who wish to stay in Japan for more than 90 days are required to apply for an extension of the period of stay to the Ministry of Justice (Regional Immigration Bureau) before the period of permitted stay is to expire.

https://www.mofa.go.jp/j_info/visit/visa/short/novisa.html