We often refer to House Price Indexes (HPIs) in discussions re house prices but there are several out there so which to use?
If you want to know average asking prices use the Rightmove HPI and the price will be for the entire-market in England & Wales.
If you want to know average selling prices it's either the Halifax or Nationwide HPIs but they only have data on buyers who have mortgages approved with them in the UK, so cash buyers for example will be excluded.
The Office for National Statistics HPI is arguably the most accurate as it publishes sold prices on all transactions in the UK (mortgages & cash). However, there is a delay so the data is at least a couple of months behind.
Anacdata HPI also covers mortgage & cash transactions and is the first to report regional monthly selling prices, but only covers England & Wales.