Here is an excerpt from the ONS website, the United Kingdom's Office of National Statistics:

"Firstly, using an internationally agreed framework, we have carefully selected over 700 items that form a virtual shopping basket. These are representative of the goods and services that consumers typically spend their money on. Each month, we ensure each item is either identical or comparable to those collected the previous month. They include many everyday items such as milk, tea, and bread, but also big-ticket items such as buying a car, as well as services such as renting your home, booking a flight, or going to a music concert. Then, prices of these items are collected physically in 141 locations by around 300 people across the country. In addition, numerous price quotes are collected online or over the phone, adding up to around 180,000 price points overall each month. On top of this, around 300,000 rental prices from across the country also feed into the headline number. We keep a close eye on the basket to ensure it remains representative of household spending habits across the country by drawing on a wide range of market research data, as well as feedback from our price collectors about which items have the most shelf space in shops. In addition, we use spending information taken from the National Accounts and a large survey of households' outgoings to ensure items receive the correct weight in the overall figures. However, it is worth noting that, while our inflation indices are representative of spending across the UK overall, they may well not totally reflect the habits of any individual consumer. Every year, we take out items which are becoming less popular and bring in new products and services. In recent years, we've taken out CD players and brought in both streaming services and smart speakers. Additions to the basket this year included hand sanitiser, loungewear, and home exercise equipment, reflecting the impact of the pandemic on consumer spending."

This explains how the ONS computes and reports:

A. The GDP deflator
B. The unemployment rate
C. The CPI
D. The nominal GDP
E. The real GDP