Isn't there a set in the core game box?
There is a full set in the box (8 base dice, 6 ammo dice and one hit location dice). An extra set of dice contains, 4 base dice (one of each), 4 ammo dice and 1 hit location dice.
Dice sets are usually are expensive but can also be popular, So the dice sets that has been delivered to various retail may be sold out there already or it can be shipping problems to restock. It can also be so that the UK warehouse have them but not the one in US and it takes time to move things across the pond these days. People could ask, using the contact form in the webbshop.