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The Direct Airside Transit visa will not allow you to pass immigration or change airports. Only some nationalities require such a visa to transit the UK airside (not passing immigration); many others can transit the UK airside without a visa.
If you need to change airports, the Visitor in Transit visa will allow you to pass immigration to get a hotel and/or ...
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You no longer need a transit visa since 2020-07-01, because Russia and Belarus now recognise each others visas. See also this post about border crossings between Russia and Belarus for more details.
Warning as of 2020-07-07: another source in Russian, dated 2020-07-01, states it will take several more months until it actually takes force. These sources ...
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You don't need a transit visa anymore since 2020-07-01.
As of 2020-07-01, Russia and Belarus recognise each others visas. That means you can use your Russian visa all the way to the Belarus-Poland (or -Latvia, -Lithuania, -Ukraine) border. See also this answer about the Belarus-Russia border.
Warning as of 2020-07-07: another source in Russian, dated 2020-...
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