If you are selling to private clients in single units or small quantities, international transport costs are always likely to be relatively high, compared to domestic UK rates. Also, if your pieces are long, or awkwardly shaped, some small parcel carrriers may not be interested, as they often have automated handling systems which may impose limitaitons on sizes or dimensions of pieces.
The large players in the small parcel market (ie the household names) will offer discounts on international tariffs, but usally only based on a regular volume of consignments, which may be a problem for an occasional exporter. There are also a number of companies who act as sales agents or whoelsalers of small parcel services, and who are often more willing to offer reduced rates for relatively small volumes of business.
If you have regular levels of business within a specific country, you might be able to investigate the feasibility of setting up a warehousing facility using a local warehousing or transport company. This might allow you to ship larger volumes to the warehouse (cutting down on your unit cost for the international journey), and allowing you to do a local delivery to your client from the warehouse, reducing costs and speeding up the delivery time to your client.
You might also consider working with a sales distributor in that country, who would buy in bulk from you, and resell in that market. This would reduce your costs, but also your profit margin.