The best way to do this is to add the grouping of regions to your raw data so, add a column next to region, calling it, lets say, segment and then put in next to each of the entries the relevant one so for all surrey, Hants and Sussex entries you add Southeast in that column. (If you have a large number of rows, you can do this quickly by creating a lookup table and using VLOOKUP to get the correct segment name.
Now add that column (segment) to your pivot table above region. The pivot table will now automatically give you a subtotal by segment.
post back if that's not clear or you can't get it to work or its not what you want.