The distribution of Formula Grant funding takes into account a measure of the usual resident population, and does not use estimates of migrant numbers separately.
The Department for Communities and Local Government uses sub-national population projections and the mid-year estimates that are published by the Office for National Statistics (ONS). These statistics take into account changes in the population through natural change (births and deaths) and internal migration (movements between local authorities) as well as international migration.
Questions on the accuracy and calculation of the population statistics are a matter for the ONS.