London asks one question of your suitcase before any other: what happens when it rains at four in the afternoon and you have dinner at eight. The answer is a trench coat, and everything else in the bag is chosen around it. Start there and the rest of the packing gets easy.

The trench is a British invention, built for weather that cannot make up its mind. Thomas Burberry founded the house in 1856 and invented gabardine in 1879, the breathable weatherproof cloth the coat is still cut from, and the garment took the shape we recognise in the First World War. The house names still set the terms: the Kensington long and belted, the Waterloo relaxed and best in black, the Pembury shorter for anyone who lives in taxis. Around them sit the quieter versions, in cotton, in wool, in nylon that folds into nothing.

What you need to decide is length and weight. A long trench looks best over trousers and a heel and does nothing for a mini. A short one is easier on the Tube. Gabardine sheds a drizzle. Wool holds warmth but sulks in real rain. Buy for the weather you will actually get, which is drizzle.

Daytime in London is a walking city dressed for a cold room. Meetings are overheated, streets are not, and the whole day happens outdoors between the two. So the daytime uniform is a wide-leg trouser, something with a collar, a fine knit that fits under a coat, and one blazer that can stand in for the coat indoors.

Wide-leg wool is the workhorse. It moves, it does not crease sitting down, and it hides a flat shoe or a low heel without any styling effort. Pinstripe reads more deliberate than plain black and costs nothing extra in effort. Under it, keep the knit thin. A bulky sweater and a trench are a fight, and the trench always loses.

A note on logos, since London wears them lightly. One is fine. A Burberry check blouse under a plain coat, a small monogram on a turtleneck, a horsebit on a loafer. The rule is that the logo should be the only one in the outfit. Two competing houses on one body reads as luggage, not dressing.

Evening in London is short on distance and long on rooms. You are rarely more than a twenty minute cab from dinner, which means you can wear something genuinely delicate as long as the coat over it is serious. This is the whole trick, and it is why the trench earns its place twice in one day.

Satin does most of the work. A silk-blend midi, a ruched silk column, a bias skirt with a lace trim: all of them read as effort while requiring almost none. A lace bodysuit under high-waisted trousers is the version for anyone who does not want to think about a dress at all, and it packs into a corner of the case.

The practical bit is lining and light. Satin shows everything, so plan the layer underneath before you plan the shoe. Anything unlined will cling in a warm restaurant. And check the piece under a lamp rather than daylight, because a black satin that looks navy in a shop window will look navy all night at the table too.

London is a walking city that pretends it is not. Cobbles in Mayfair, grates in Soho, escalators everywhere, and the distance from the Tube to the restaurant is always longer than the map suggested. Daytime footwear has to survive all of it, which rules out anything you would hesitate to put on wet pavement.

The answer is a boot with a real sole or a flat with structure. A knee-high in leather goes under a long trench and over a trouser and disappears into both. A low block heel on a suede ankle boot gives height without commitment. Loafers and ballet flats work if the sole has some substance, and they pack flat, which matters more than it sounds.

For bags, one that carries a folded umbrella and closes properly. Suede is the risk here. It looks beautiful and it marks in rain, so it belongs on the dry days and the indoor ones. Nubuck and treated leather are the safer buy if the trip is short and you only get one bag in the case.

After dark the shoe gets thinner and the bag gets smaller, and that is the entire change. Nothing else about a London evening requires a costume. A slingback and a clutch turn the daytime trousers into dinner, and you have carried two small things instead of a second outfit.

Heel height is the real decision. Seventy millimetres is the number that works on a London pavement and still reads as evening. Anything above a hundred is a taxi shoe, which is fine if you are going one place and coming back, and miserable if the night moves. A slingback is more forgiving than a court because it does not need to grip.

The clutch should hold a phone, a card, a lipstick and a key, and it should close without being held shut. A soft leather pouch is easier to pack than a structured frame and looks better crushed. If you are choosing between a beautiful clutch and a practical one, the practical one wins, because you will be holding it for five hours.

The best thing in a short case is the piece that does not care what time it is. A check shirtdress with a belt goes to a museum at eleven and a wine bar at nine. A printed silk midi does the same in reverse. A satin dress in a dark colour is a lunch dress in a cardigan and a dinner dress without one.

Accessories are how you move a look across the day without repacking. A scarf changes a coat completely and weighs nothing. A good belt changes the proportion of a dress. A slingback in a print does more than a plain one and takes up the same room. This is also the sensible place to spend on a logo, because an accessory carries it more quietly than a garment.

Build the case backwards from this. Two pieces that work twice, one trench, one day shoe, one evening shoe, one bag that closes. That is four days in London with room left for whatever you find on Mount Street.

Buy the trench and stop optimising the rest. It is the one purchase here that will still be in rotation in ten years, and a good one at fifteen hundred does the same job as a spectacular one at four thousand. Where we would not save is the day shoe, because a bad boot ruins a London trip in a way that a cheap sweater never will.

We would skip the very high heel entirely. We would skip suede if you are going in autumn. And we would be honest that the high street trench is a real answer, not a consolation. The seventy dollar version photographs almost identically and you will not grieve it when it gets soaked.

If you take one thing away, hold the coat and the evening dress up together in the shop before you buy either. If the coat cannot go over the dress, one of them is not coming on the trip.