Any question answered...

But what are we talking though, on a parcel worth £150? If it was ten quid I wouldn't mind paying (too much), but if I stand to get bent over a barrel and rogered into the middle of May then I'd rather not bother placing the order at all.

Assume 10% duty on the goods and then 20% VAT on goods+duty+shipping, so on 150+25 you're looking at £15 duty and then £38 VAT, and then the fucking post office charge you £13 for collecting the taxes, making a grand total of about £61 to pay before they hand over your parcel. Duty rates vary slightly according to the type of goods, but duty is the smaller part of the problem anyway.