I may need to go to the UK for a day. I live in Turkey and I am a Turkish citizen. Do I need a visa to come to the UK for only one day?
The IATA has an online resource to answer these type of questions. It seems (after entering some random data), that you need a visa even for 1 day.
When visiting the UK, use the UK Border Agency “do you need a visa?” questionnaire.
Being a national of Turkey, you do need a visa almost no matter what. If you stay in the UK for one day on your way between two different places, you'll be in transit (transit can be up to 48 hours for Turkey, shorter for some other nationalities). UK transit comes in three flavors: airside (staying in an airport without passing through immigration), landside, and “common travel area” (i.e. on your way to or from Ireland, the Channel Islands or the Isle of Man). There are rare and complex transit visa exemptions, mostly to do with visa status in the EU, US, Canada, Australia or NZ. Only airside transit has special rules, other types of transit visa fall under the general heading of visitor visas.
Coming from Turkey, you will need to apply for a UK visa through TLSContact (a commercial company that manages all UK visa applications in many locations).