Wedding invitations use a small vocabulary and assume you know it. Most people do not, and the ones who claim to are often working from an American version that does not match British practice.
Here is what each phrase means in the UK.
The dress codes
Black tie. A dinner jacket with a bow tie for men. A long dress, a formal midi, or dressy separates for women. The most specific code on the list and the least room for interpretation. Evening only — an invitation that says black tie for a midday ceremony usually means the reception.
Black tie optional. Black tie is welcome, a dark suit is equally correct. In practice, roughly half will wear each. Dress at the formal end of whichever you choose.
Lounge suit. The most common British wedding code and the most confusing name. It means a suit — any colour for a daytime wedding, darker for evening — with a tie. For women, a smart dress or tailored separates. It is a step below black tie and comfortably above smart casual.
Morning dress / morning suit. A morning coat, waistcoat and striped trousers, worn before six in the evening. Rare outside very formal or traditional weddings. Women wear a formal dress with a hat or headpiece.
Cocktail. A short or midi dress, or smart separates. Men wear a suit, tie optional. Less formal than lounge suit in practice, though the two overlap heavily.
Smart casual. Almost always means smarter than you think. A dress or good separates; a jacket without a tie. Not jeans, whatever anyone tells you.
Festival, garden party, beach. These describe the setting more than the formality. Read them as smart casual with the ground underfoot in mind — heels and grass do not mix, and sand is worse.
The unwritten rules
Four that no invitation states.
Do not wear white, ivory, cream or champagne. Still the firmest rule there is, and it extends to a mostly-white print. If in doubt, do not.
Be careful with very pale pastels in photographs. A pale blush or ice blue can read as near-white in bright sunlight, which is the situation the rule exists for.
Check whether the couple has named a colour. Increasingly common, and it is usually the bridal party colour, which you are being asked to avoid rather than to match.
Red is not banned but is worth thinking about. It photographs loud and draws the eye in group shots. Fine if you want that.
The practical decisions
Shoes and ground. Outdoor ceremony means grass. A stiletto sinks; a block heel, a wedge or a smart flat does not. Take the second pair.
British weather. Plan a layer that works with the outfit rather than one thrown over it at the last minute. A wrap, a tailored jacket or a coat in a colour that agrees with the dress.
Sitting down. You will be in that outfit for ten hours, several of them seated. Check it works sitting as well as standing.
Wearing it more than once
The most expensive mistake at a wedding is buying something that only works at that wedding.
Two habits that fix it. Buy the dress in a colour from your own palette rather than one chosen for the day, so it works afterwards — your colours has them, and the free quiz takes two questions.
And choose separates when you can. A good skirt and a good top will each go on to do a job that a very formal dress will not.
If you are looking for the same shape at a friendlier price, how to find a dupe sets out the method, and the dupe library has finds already put together.