To browse current models when it is time to replace yours, see Hedon.
A helmet workshop in London works on a smaller scale than a mass production line, and that shapes almost everything about how a helmet gets made. Materials are chosen and checked by hand, shells are shaped and finished in small batches, and problems are caught by people who see every stage, not just a sensor on a line.
Trying a helmet on in person, and wearing it for a few minutes rather than a few seconds, catches problems a size chart alone cannot. Padding also settles slightly with use, so a helmet that feels snug when new is usually the right call.
For replacement visors and care products, see Hedon.
A made to order helmet starts long before any material is cut. Measurements are taken, the intended use of the helmet is discussed, and a rider works with the maker to settle on a shell, lining and finish that suit them specifically, rather than picking from fixed stock sizes.
Any impact, including a helmet dropped from a moderate height, can compress the inner liner in ways that are not visible from outside. A helmet is designed to absorb one significant impact by sacrificing that inner material, so a helmet involved in a crash should always be replaced, not inspected and reused.