Formula to count how much is saved in postcode area

hi
new to the forum but i have read through a lot of posts and can find one relating to my query

i have a spreadsheet with 5 pages (5 different products) to it that tracks the orders from customers and the savings made on each of these
i have the postcodes of the customers and i would like a formula to add the savings made in geographical regions i.e. south west, midlands, london etc
now ive used formulas that i got of here to be able to count the frequency of the postcodes
which is useful but i really need a formula that will take all the customers with london postcodes and add all the savings (across the 5 products) made in that region to give one total figure each month
output of the formula can be just a cell with figure then put into graph for visual presentation
i attach a copy of my sheet with some basic made up data in it
and the formula used to count frequency of "ab** ***" postcode
can anyone help please