Armour Wearing - It is forbidden for MPs to wear suits of armour in the House of Commons - apparently it hinders the process of duelling. It is also illegal to die in the Houses of Parliament - since anyone dying in Parliament would be legally entitled to a state funeral.
Scots Injustice - In the city of York, it is legal to murder a Scotsman within the ancient city walls, but only if he is carrying a crossbow. Except on Sundays when he can wander around with impunity.
Beached Whales - Any whale discovered on the British Coast, must be offered to the Crown. The head belongs to the King and the tail goes to the Queen for use in making her corsets. Unfortunately, corsetry was made using the baleen plates, which are actually found in the head. So this law was of no practical use when launched in the 14th century, but has somehow survived. In reality the Natural History Museum is contacted when whales become beached.
Bovine Driving - One may not drive a cow while drunk. The Licensing Act of 1872 explains that operating a cow while intoxicated carries a prison sentence or a £200 fine.
Pelican Touching - Section 23 of the London, Royal and Other Parks & Gardens Regulations of 1977 states that 'Touching a pelican' is expressly forbidden, unless prior written permission is obtained. Presumably from the park, rather than the pelican.
Pensioner Impersonating - Dressing up as a Chelsea Pensioner is illegal in the UK, due to their status as national treasures.
Hay Carrying - London Taxis must carry a bale of hay and a sack of oats. The London Hackney Carriage Laws changed little since their introduction. The oats and hay were for the horse, not the driver. Disputes have often arisen, and some firms did manufacture tiny bales of hay, so that taxi drivers could remain within the law when they first became mechanised. (The act was finally repealed in 1976).
Stamp Inverting - It is an act of treason to stick a stamp bearing the British Monarch upside down (or to tear it).
Mince Pie Imbibing - A law from Puritan times in the 17th century, still states that eating mince pies on December 25th is illegal.
Unlicensed Alien Invasion - Magistrates are permitted to use 'reasonable force' to prevent an alien invasion of the UK - as long as the aliens don't possess a licence to invade. From section 9 of The Outer Space Act 1986. One senses they had too much time on their hands in 1986.
The Royal Pet's Fidelity - A law passed during the reign of George I states that: "The severest penalties will be suffered by any commoner who doth permit his animal to have carnal knowledge of a pet of the Royal house." Bear that in mind if you run into any royal corgis up the local park.