Calculation is not correct during in the left join table
"Explode-Implode" syndrome causes the inflated SUM.


The JOINs are done.  This gives an intermediate table with lots of rows.
SUM() and other aggregates are performed.
The GROUP BY shrinks it back down to a small number of rows.


Where practical, this is a convenient solution; note that there is no JOIN or GROUP BY:

SELECT 
        ( SELECT SUM(...) ... ) AS sum_1,
        ( SELECT SUM(...) ... ) AS sum_2
    FROM ...


But that won't work here.  Instead, let's compute the aggregates in "derived" subqueries:

SELECT COALESCE(uk.qty, 0) AS QTY_sold_in_uk,
       COALESCE(de.qty, 0) AS QTY_sold_in_de,
       ...
    FROM  tbl_inventory AS ti
    LEFT JOIN ( SELECT sku, SUM(qty) AS qty ) AS uk  USING(sku)
    LEFT JOIN ( SELECT sku, SUM(qty) AS qty ) AS de  USING(sku)


This way, the aggregation will be confined to the the one table.  Also, the LEFT allows for an item not to be sold in some country, then the COALESCE deals with the NULL.  Again, no GROUP BY.

This pattern begs for at least 5.6 because the derived tables have no index.  However 5.6 will dynamically build one.

Final form

SELECT 
    ti.sku,
    ti.quantity,
    -- UK
    COALESCE(suo_uk.sku, 'Not Sold') AS Unit_sold_in_uk,
    COALESCE(suo_uk.units_ordered, 0) AS QTY_in_uk,
    -- De
    COALESCE(suo_de.sku, 'Not Sold') AS Unit_sold_in_de,
    COALESCE(suo_de.units_ordered, 0) AS QTY_in_de,
    -- Fr
    COALESCE(suo_fr.sku, 'Not Sold') AS Unit_sold_in_fr,
    COALESCE(suo_fr.units_ordered, 0) AS QTY_in_fr,
    -- ES
    COALESCE(suo_es.sku, 'Not Sold') AS Unit_sold_in_es,
    COALESCE(suo_es.units_ordered, 0) AS QTY_in_es,
    -- It
    COALESCE(suo_it.sku, 'Not Sold') AS Unit_sold_in_it,
    COALESCE(suo_it.units_ordered, 0) AS QTY_in_it
FROM
    tbl_inventory AS ti
    LEFT JOIN ( SELECT sku, sum(units_ordered) as units_ordered
                     FROM tbl_sku_units_order
                     GROUP BY sku ) AS suo_uk
                             ON suo_uk.sku = ti.sku
    LEFT JOIN ( SELECT sku, sum(units_ordered) as units_ordered
                     FROM tbl_sku_units_order_de
                     GROUP BY sku ) AS suo_de
                             ON suo_de.sku = ti.sku
    LEFT JOIN ( SELECT sku, sum(units_ordered) as units_ordered
                     FROM tbl_sku_units_order_es
                     GROUP BY sku ) AS suo_es
                             ON suo_es.sku = ti.sku
    LEFT JOIN ( SELECT sku, sum(units_ordered) as units_ordered
                     FROM tbl_sku_units_order_fr
                     GROUP BY sku ) AS suo_fr
                             ON suo_fr.sku = ti.sku
    LEFT JOIN ( SELECT sku, sum(units_ordered) as units_ordered
                     FROM tbl_sku_units_order_it
                     GROUP BY sku ) AS suo_it
                             ON suo_it.sku = ti.sku
    GROUP BY ti.sku