Yes this is a problem in London at the moment but also keep it in perspective. Think how many millions of people are using their phones out and about everyday without a problem.
‘Traditionally’ this type of theft has commonly happened outside of tube stations. People wander out distractedly staring at their phones to look at a map or contact friends. It also happens on quieter streets, where people might feel safer and the thieves have an easier and faster exit.
If the thieves can’t see your phone they can’t steal it.
Don’t wander around within your hand.
If you need to use it step way from the kerb and stand by the wall side of the pavement. To be honest this is also the polite thing to do so you aren’t blocking the path of other people.
I personally am not a fan of lanyards and anti theft bags. If someone tries to snatch something from me at speed I want them to take it. I do not want to be dragged along by them. This is based on the experience of a friend’s mum when someone tried to snatch her bag in Barcelona. She was dragged down the street and ended up in hospital with broken bones. If you do choose to use a lanyard etc at least choose one where the strap can snap. Not just because of thieves but also in case they become caught for example in a bus or tube door.