Anna Harvey on the best wedding guest outfits to buy off-the-peg

I wonder how many guests at The Wedding ten days ago bought their outfits off the peg? By that I mean that they went into a shop and found something they loved, rather than borrowing an outfit from an international designer, as a celebrity might, or having something made.

I know one lovely young guest who found her outfit at H&M and wore it with the obligatory hat and platform espadrilles. But you need confidence to do that. (She also happens to have her own website, Maudiecollections.com, selling some great jackets, though not probably suitable for a wedding at Windsor Castle.)

What you buy depends on what sort of wedding you’re going to. If it’s a traditional church wedding then certain looks work better than others. Anything strapless, very short or showing too much flesh is probably best avoided unless accompanied by a jacket. Over the years of attending church weddings I have noticed that outfits worn with a hat in a matching colour work best. Lady Sarah Chatto looked particularly good at the royal wedding in navy blue Jasper Conran with matching Stephen Jones hat: a lesson in how played-down simplicity often works best.

Jacquard weave dress, ￡49.99, H&M; J by Jasper Conran wrap midi dress, ￡55.20, Debenhams; Diane von Furstenberg yellow printed silk dress, ￡450, Net-a-Porter 

My advice would be to select your outfit first and then find a hat. It shouldn’t be necessary to have a hat made to match as most department stores have excellent selections of designer label options. Try Debenhams, Fenwick, John Lewis and also specialist boutiques like The Hat Gallery and Herald and Heart. Your shoes and bag also have to match - in which case neutrals or metallic may work best.

From my viewing position on the sofa, I thought many of the guests looked wonderful: the Countess of Snowdon in vanilla Carolina Herrera and her daughter Margarita in Paule Ka both looked lovely. Cressida Bonas in rainbow bright Eponine also looked so pretty. The groom’s aunt Lady Jane Fellowes (who read from the Song of Solomon so beautifully) chose a silk outfit from shibumi.co.uk which she looked perfect in. Possibly because she is not a regular online shopper, she travelled from London to Stroud to choose something from the bricks-and-mortar shop there. She found a simple brocade coat-dress and was very happy with her choice.

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I only once had something made for me for a wedding, by Vivienne Westwood, and was delighted with it. For other family weddings I have always bought off the peg. I like to try things on and feel the fabric rather than buying things online to try at home and most likely return. Good ready-to-wear brands for smart occasions include Red Valentino, Iris & Ink, Bruce Oldfield for John Lewis, Michael Kors, Diane von Furstenberg, Caroline Charles, Max Mara, DKNY, Cefinn, Vivienne Westwood Red Label, and J for Jasper Conran at Debenhams.

Mosaic print dress, ￡465, Eponine; MICHAEL Michael Kors tiered maxi dress, ￡247, Farfetch; Sleeveless floral dress, ￡595, Caroline Charles 

If in doubt, opt for professional help. Many of the bigger department stores have personal shopping services where you can contact the store ahead of your visit and explain what you need and what sort of things you like so by the time you arrive in store, they will have pre-selected a rail of clothes for you to try. Harvey Nichols, Selfridges and most big stores now offer this service.

But my best advice is to visit your favourite shop with a trusted friend or, even more reliable, a daughter if you have one. The latter has no reason at all not to be completely honest, and will be!

Meet The Telegraph's Timeless Style columnist, Anna Harvey: the Ex-Vogue deputy editor, consultant to Princess Diana and author of Timeless Style: dressing well for the rest of your life (￡12.89, Double-Barrelled Books).