London has the best 4G coverage in the UK, but also the slowest speeds. That's according to a new State of Mobile Networks report from Which? and OpenSignal that has broken down the key 4G stats for the various regions in the UK. According to the report, mobile phone users in the capital are able to access a 4G network 69.7% of the time. In Wales, that figure falls to a meagre 35.4%.