Sent stuff back a few times; once it got through, twice it didn't. On the two that didn't, one of them resulted in a letter from HMR Customs & Excise, basically telling me that they had my weed and I was more or less welcome to come and collect it from them, but they'd arrest me for doing so.
So, in my experience, there's minimal risk of anything actually bad happening (unless it's silly amounts), but there's also a reasonable chance that you won't get it.
If it's hash, just hide it in your luggage and fly into a small airport (Bournemouth's great and comically undermanned!)
