Want a larger bush for dry spells? The obvious picks are not all equally tough

For your exact question, the strongest larger flowering shrubs for dry periods are Cistus, Buddleja and then Ceanothus. They all cope with sun well, but they are not equal: Cistus is usually the most dependable through prolonged dry weather, Buddleja is very forgiving and easy, and Ceanothus is best where the soil is free-draining rather than heavy or wet in winter.

Which ones best fit a larger, low-maintenance bush

• Cistus: one of the best for heat, poor soil and long dry spells once established. It stays fairly low-fuss and flowers well in full sun.
• Buddleja: a good choice if you want a bigger presence quickly. It tolerates dry conditions well and is easy to keep going, though it can get large and needs occasional cutting back.
• Ceanothus: very good in a sunny, sheltered spot with sharp drainage. It can make a handsome larger shrub, but it is less forgiving than Cistus if the ground stays wet.
• Lavender: excellent with drought, but usually not what people mean by a larger bush.
• Santolina: very tough and low maintenance, but also on the smaller side rather than a substantial flowering shrub.

Because you said you would like a larger bush, I would lean first to Cistus if your priority is pure toughness in dry periods, or Buddleja if your priority is size and ease. Ceanothus is worth choosing when you want the look and your site is sunny with well-drained soil.

At this point in late August in the UK, you can still choose and prepare the spot now, but planting is usually better as the weather cools and the soil starts to hold more moisture in early autumn. Give whichever shrub you choose a thorough first season of watering so it establishes properly; after that, its dry-weather tolerance matters much more.