Britain's current terror level is "severe". That means, according to the government, an attack is "highly likely". It's just one notch below the highest level, when an attack is "expected imminently". This might seem shocking but it's useful to bear in mind that Britain's terror level has been "severe" since January.
When put in this context, the hysteria surrounding the French government warning its citizens to be extremely vigilant when travelling in Britain is a complete non-story. France warned its citizens that a terrorist attack in Britain was "highly likely". In other words, nothing has changed. France gave its citizens the same advice that the UK has been giving its own citizens for the past six months.
Long before the weekend, a terrorist attack in Britain was "highly likely". The French government now agree. That's it. Nothing else has changed. Nothing else is new. It's not news.