I haven't used them (yet) and they're presently out of stock of buffalos, but Sarnia Silver in in the Channel Islands will send you one of these for £14.50 including delivery to the UK. Not a bad deal when you'd be hard pressed to get a silver britannia from ebay for less than £20 delivered. They do lots of other coins and bars too.
The main disadvantage compared to the silver britannia is that the buffalo potentially has CGT payable upon sale (assuming you've used up your annual CGT limit, etc). All foreign silver coins have this disadvantage, of course.
There was a thread that mentioned Sarnia on here recently. Apparently the way they work is by sailing under the £18 VAT threshold per item coming from the Channel Islands. This means that if you order 10 coins, you'll get 10 envelopes.
As long as they can profitably send an ounce of silver plus postage for less than £18, they'll have a great business. Depending on who you believe, you may need to hurry
cheers
sjiff