True, sort of, certainly if the rise were to happen today
50p in 2018 is equivalent to 62.4p now
Source: https://www.bankofengland.co.uk/monetary-policy/inflation/inflation-calculator
However inflation in the 3 years between 2015 and 2018 was equivelant to about 8 months worth of inflation last year
If the rise is going to be rolled out in September that's 7 months away and we don't know what the rate of inflation is going to be between now an then so there is some doubt about what the inflation rate over the whole period would be
If inflation were, for example, to hold constant at the December rate of 5.2% for 7 months then by September that 62.4p would be 64.3p
You can, of course, speculate about how inflation is likely to be a little bit lower but possibly we need to calculate for 8 months given that the 62.4p would have been a January number (that takes it up to 64.5p)
But the general point, that in real terms this price rise will be roughly equivalent to inflation, stands up pretty well