In my experience, it depends on the courier used. Some couriers automatically apply the duty (often with a handling fee) to all packages where the declared value is over the limit - I think it is a way of earning revenue for them through the fee charged. I recall once being charged duty + a £10 fee by UPS for a couple of Region 1 DVDs coming from Amazon in the states where the identified value was loose change over the £18 limit at the time. Sad to say, the driver who demanded the fee was not over happy at the response he got... Other couriers never apply the duty. I can't recall anything which came via the Royal Mail ever having duty charged.

I don't recall ever paying duty on anything from PG - perhaps those checking the parcels are particularly nice to things coming from Canada rather than the US!!

Brian