The UK’s biggest supermarkets were overwhelmed with customers flooding their online shopping platforms to book delivery slots, following the government’s announcement of a third national lockdown. Prime Minister Boris Johnson confirmed the national lockdown on Monday at 8 p.m., and within hours shoppers were reporting problems with supermarket websites. Many shoppers were frustrated to learn that Ocado’s online shopping website had a virtual queue of more than 10,000 people. Sainsbury’s temporarily limited access to its online groceries service on Monday night to “manage high demand for slots and updates customers were making to existing orders,” a spokesperson confirmed. “We’re continuing to monitor the situation and are sorry for any inconvenience this may have caused.” Morrison’s online booking for grocery deliveries showed “no more slots available” for some areas at the time of publishing. Asda said they saw a spike in visits to their website after the lockdown announcement. A spokesperson …