I work in a Post Office and can tell you that in July the Dangerous Goods Act came into force. This started in January for goods going abroad, but in July for inland parcels. This has come about because a lot of inland post goes airmail (for example if you are sending something from Liverpool to Exeter that would go by air) and the Civil Aviation Authority has just granted Royal Mail (and no other carriers) a licence to carry small amounts of dangerous goods by air. Dangerous goods can include perfume and nail varnish (both flammable) and lithium batteries (apparently these can set on fire if crushed) as well as other item. Post Office staff now have to ask what are in parcels (or packets, or large letters if obviously goods and not just a letter) to ascertain if they contain a dangerous good. It doesn't mean that you may not be able to send the parcel, sometimes the staff have to put a sticker on the parcel to warn that it contains small amounts of dangerous goods. You might find that staff do not ask every customer, this would be because that customer might be a regular customer and they know exactly what they are sending (eg they may sell handbags). The CAA are going to be sending mystery shoppers into Post Offices to check whether the staff ask what is in the parcels (and we don't need detailed information just the basics eg it's a book, not it's Harry Potter and the Philosophers Stone!). If staff are found not to be asking the question then the CAA have the authority to stop that particular Post Office from taking in mail, which is a huge amount of work that Post Offices do. You may be angry now, but you would be a lot angrier if a plane crashed on your house because something in the hold had set on fire. This is basically what it is all about, safety.
And yes, you can lie, but all parcels are being scanned at the sorting offices and if they are thought to contain dangerous goods then the parcel will be destroyed and you will only be informed of this if you put a return address on the back of the parcel.
And please remember, Post Office staff don't make these rules up (the bosses do that!), and they don't ask you these questions to annoy you, they ask the questions because that is what they are told to do.