Build quality matters because this is a heavy cat
A British Shorthair is a dense, cobby cat who weighs far more than his plush outline suggests, and a carrier that flexes under him feels unsafe to both of you. Look first at the base: it should be rigid, stay flat when lifted by the handle with the cat inside, and carry a stated weight limit comfortably above his actual weight. Reinforced corners, a handle anchored into the structure rather than clipped onto thin plastic, and latches that cannot spring under a shifting load are worth more for this breed than any padded extras or fashionable shape.
A wide, low door for a cat who will not be folded in
British Shorthairs are famously undramatic but quietly stubborn, and a broad, dignified cat does not appreciate being posted through a narrow opening. Choose a wide front door he can walk through, or better, a top-opening design that lets you lower him in without negotiation, which also lets a vet examine him in the carrier base. A low entry lip helps, especially for an older Shorthair with stiff joints, and the interior should let him sit upright in his natural compact loaf and turn around without his dense coat brushing every surface at once.
Ventilation without draughts for a placid traveller
This breed usually travels calmly if the basics are right, so prioritise steady airflow through panels on at least two sides, without a direct draught across the cat, and a design that stays cool in a warm car, because that thick double coat insulates in both directions. A cover to dim the carrier helps a Shorthair who copes by sitting quietly and waiting it out. Line the base with a non-slip, absorbent pad under familiar bedding, since a heavy cat on a sliding mat feels every corner the car takes and will brace, tense and arrive exhausted.
Comfort details matched to a sitter, not a climber
Unlike athletic breeds, a British Shorthair will mostly sit or lie in one chosen position, so cushioning quality matters more than interior height beyond standing room. A firm, washable cushion that does not bunch under his weight, room for a small water dish on longer journeys, and a smooth interior with no ridges digging into a broad back all count. Practise short, boring carrier sessions at home with treats, because this breed's cooperation is earned through routine: a carrier that lives in the living room and occasionally produces chicken is a carrier he will enter without a household summit meeting.
Check the carry, not just the carrier
With a heavy Shorthair, how you transport the carrier matters as much as its label: two handles or a design you can support from underneath with both arms will save your back and his dignity, and the carrier should belt securely into the car so it cannot tip. Shoulder straps on soft carriers are a poor match for this breed's weight over any distance. Keeping his weight record and vet history up to date in his KinsLe Pet Passport helps you check the carrier limit honestly as this slow-maturing cat fills out.