Sitefive
Perhaps you should ask the private couriers in your country what method they use for delivering to the UK and where in the UK they deliver to.
It seems to me that they have two options:
a) Load a van/lorry with all their deliveries to the UK, drive to a Channel Port and then drive to the delivery destination although 1,800 miles is quite a way to come. In this case, their costs are driver’s pay, fuel, toll charges if going via France and Italy, ferry costs plus hotel expenses. Their costs may also depend on how far they have to travel in the UK from the ferry port. Where do they normally deliver to? Delivering from Dover, say, to addresses in Kent is substantially cheaper than delivering to Northumberland, Cumbria or Scotland. Similarly, if they arrive at a port in the north of England, Kent, Cornwall etc. are not so easy to get to. However, if they have already travelled 1,800 miles, an extra 200-300 may not matter. Presumably, compared to the UK, their driver's pay (plus add-ons such as social insurance, pensions etc.) isn't that much and they may not have to pay VAT on their charges. Finally, do they only deliver once they have a full van load.
b) They only collect in your country and then use other transporters (truck/rail/sea) to deliver to the UK or else they deliver to the UK themselves. Do they then get a local, UK, firm, to deliver within the UK. In this case, why not ask them who they use for the final miles? If their costs in delivering to the UK are the same as you can find at the moment within the UK, if you deduct their costs of delivering to the UK in the first place, the UK elements of their costs ought to be cheaper than anything you have found so far.