Change cell value based on another cell's value

I know it sounds a lot like conditional formatting, but I promise you it's not.

I have two columns of data, one for 'District' and one for 'Postcode'. I want the value of a cell in District to change based on the content of a cell in Postcode, e.g. if the postcode is NH then the adjacent cell in District would change to North Scotland; if the cell is SE it would change to South England etc. I need the formula to be able to be used for an entire column and workbook, simply changing colour isn't enough as I need to be able to filter it by district.

Any help is much appreciated!

Sample spreadsheet attached.

So I need the values of the cells in column A to change automatically based on the values of the cells of column B according to the values table in columns D & E.

Hope this works, used =INDEX($E$3:$E$8,MATCH(B2,$D$3:$D$8,0)) in cell A2 and dragged down, change the value in column B and it will change in column A to match the data in the table