For the time period of November 1 to November 28, eCommerce accounted for 15% of retail sales. Notice that the data did not include Cyber Monday:

Online sales made up 15.2% of total sales in November, the highest share since records began in January 2008, according to the Office for National Statistics. This year, consumers spent 1.1 billion pounds ($1.64 billion) on Black Friday, up 35.8% from 810 million pounds in the same period a year ago, according to data from IMRG and web measurement firm Experian.

Also note the dominance of Amazon as the commerce point of entry:

And when Britons do shop online, they increasingly turn to Amazon.com Inc., No. 1 in the Internet Retailer 2015 Europe 500. A BloomReach survey of 1,000 U.K. consumers on holiday shopping behavior finds that 90% of them will consult Amazon on their gift purchases this year, using it as a search engine and gift idea generator: About 43% named Amazon as the starting point when they knew what they wanted to get a person, and 41% said they turned to the online retailer as the starting point when they didn’t have a gift idea.

Amazon also ranks highly for comparison shopping. 46% of U.K. consumers will comparison-shop on Amazon for about half of their holiday purchases, and 80% of those surveyed will buy from Amazon, BloomReach says.