retail price index 
noun
(In the UK) an index of the variation in the prices of retail goods and other items.The change will not affect the payment of benefits, which will continue to be indexed to the retail price index....
- A student loan starts accruing interest from the moment it is borrowed, but the interest rate is pegged to the retail price index.
- What's more, house-price growth has easily outstripped inflation (rising prices) elsewhere, with the core retail price index up only 28% over the past ten years.