The findings, part of a nationwide analysis powered by Opensignal, show that E20 mobile users only experienced a reliable connection 74.8 per cent of the time—well below the 85 per cent or higher seen in many other London postcodes.
The study measured signal for four major network providers – EE, O2, Three, and Vodafone.
E20 covers the Queen Elizabeth Olympic Park in Stratford and developments which surround it.
Natalie Hitchins, head of home products and services at Which?, said: “This research shows just how dramatically the quality of mobile coverage can vary.
“Choosing the right provider can mean the difference between seamless coverage or constantly battling to make a call from the comfort of your own home.”
The worst postcode for mobile quality in the UK was SA70 in Tenby, Wales, where users only had a good connection 54.3 per cent of the time.
In contrast, SE11 in London—covering parts of Kennington and Vauxhall—tied for best coverage in the UK with Nottingham’s NG3 postcode, both scoring 87.8 per cent.
Other high-performing London postcodes included SW3 and SE27.
The Which? mobile network quality map is based on handset data covering the year to August 2025 and reflects the percentage of time users had a good mobile experience in each postcode.