Not sure about the UK, but here's the story in Aus:
When it gets posted O/S to Aus, the sender fills in a declaration saying the total value of goods in the package. If it's under $1000, it goes through no problems. If it's marked as over $1000 (or they randomly inspect and reckon it's worth more), then you get a letter from the Customs house saying that you can get it when you pay them the import tax. Then you either pay their bill, send a cheque, whatever, and they'll forward it on or you can pick it up.
One easy way around all of this is to buy in multiple packs, sometimes twice the shipping is still cheaper than paying the customs duty (and in Aus, all of that is still way cheaper than paying local prices).

I'm presuming the UK works very similarly, just the max-allowable value of goods will change (I think your max is 300 or something?), someone from your ashes-less end of the world can probably confirm, or it should be on the HRMC website.