Visit the UK with Schengen tourist short-stay visa
I'm Russian, and I have touristic (C-type) Schengen Schengen short stay visa. I want to know, can I stay for some period in UK without getting UK visa, or it is not possible (I heard about 36 hours free of visa staying or something like that)
UK is not part of the Schengen Agreement and therefore a Schengen visa as such does not allow you to enter the UK. According to official rules published by the UK Border Agency, Russia falls under the list of countries that require a visa before you can enter the UK. You can easily check if you need a UK visa here.
However, this only applies for longer stays. If you simply wish to transit through the UK, then you can 'transit without visa' (TWOV) with a limit of 24 hours of stay. This facility is available independent of whether you have a Schengen visa or not.
If you wish to stay longer than 24 hours, then you cannot 'transit without visa'. In this case, you will have to apply for a 'visitor in transit' visa that allows you to stay for up to 48 hours.
Note that 24h TWOV transit option is **not** available to all nationalities. Nationals of few select countries even require a UK visa for the short walk through the airport without leaving the airside (international) zone or going through immigration, the Direct Airside Transit Visa.