The savings form a joint order MAY be associated with carriage - it will depend on how the shipped wieght drops in terms of the weight ranges used by the carrier. The more expensive the item, the less significant over all.
For instance - you might get one, two, three, more items sent for the same price due to weight. Or not.
There are no other savings woth worrying about (you get one import admin charge - £12 - not really worth worrying about).
The only realistic way to chance avoiding import charges is to use postal services, NOT a courier. Most couriers (DHL certainly) have delegated authority from HMC, so automatically calculate and charge what is due.