This is an ideal application for a lookup table.
See this mock-up click here with completely made-up VAT rates.
I'm assuming that the VAT rate has always changed on 1 April.
So he would have a date column, then a column with the looked-up VAT rate, then columns for the calculation.
To keep things tidy it would be better to keep the lookup table on a separate sheet, then do the lookup like this:
The syntax is:
VLOOKUP(value to lookup, range to look in, column number in the range to find a value in, True or False)
True finds the nearest match, False tries for an exact match (we don't want that).