There's a few threads about this a bit further down. There's a code which if the seller writes on means that you shouldn't pay fees - at least up to a certain amount. If the seller marks as gift and values under $25 you won't pay any fees either.
To get away without paying any duties or tax item will have too be below £36 for gifts or £18 for merchandise (I think or roughly around these figures).
If declared at cost then its 20% in GBP plus a service charge.
I have never use that tax code, but apparently it works!!