The simple fact is that you can't know for sure.
You can ask if the source has any known bias, if the source has a history of being honest, if it has any reason to lie in this particular situation.
The Russian source clearly has a bias, it is pro-Russia! This does not mean that everything that the Russian MOD says is false.
The UK MoD has also clearly indicated its support for Ukraine, so it is a biased source. But as the UK is not one of the belligerents, it may be less biased. The UK Mod also has a history of being less than completely honest(!)
There are other news agencies, but their reporters may not be able to get first-hand intelligence, the reporter is not biased but if they are restricted, they might not get the full picture. And the situation is complex. There may be half truths, opinion-reported-as-fact, truth-but-not-the-whole-truth and outright lies in the reporting.
So what you ask for might seem simple but it isn't. There is no way to know what the truth is in any war situation in which the belligerents are willing to lie (which is probably the case in every single war).
Sometimes historians can figure out what really happened, but it may be years after the fact.