The most common occurrence is this - ebikes driving by (or even on) the pavement and snatching your phone. Grip your phone tightly if you do need to take it out and check it, but if you need to use it for a prolonged period of time, either go into a shop or at least stand close to the building (facing towards the building). Just know that if your phone is visible, these ebiking thieves will definitely drive on the pavement to get closer to you. If possible, just don't use your phone for more than a few seconds in public - or get a strap for your phone that hooks around your body.

It is a real problem, and to my knowledge (based on personal + friends experience) it seems to be particularly bad in London, perhaps the worst in the developed world. I have lots of friends here who have not had their phones stolen, but have definitely seen phones being snatched.

Tourist areas such as Oxford Street are the worst. Make sure you have iCloud back up; if it does get stolen, make sure to report it to the police immediately, not only for insurance, but there is actually some effort to catch these thieves now (particular by City of London Police) so you may get lucky and there may be active patrols nearby.