By Susanna Twidale LONDON (Reuters) – Britain’s National Grid said on Thursday there could be periods where electricity supply is tight this winter, given uncertainty over supplies of Russian gas to Europe, but that it expects to be able to meet demand. Countries across Europe are making contingency plans for winter after Russia reduced natural gas flows and said supplies could be cut further or even stop. Many European leaders believe those actions are a reaction to Western sanctions on Russia in response to its invasion of Ukraine. “While Britain is not reliant on Russian gas to the extent t…