The main thing to remember in a heatwave is to conserve water as best as possible. This can be done by a number of methods:

• Collect rain water in water butts or underground storage tanks (rain water
  harvesting) – this can be from your house, garage, shed or greenhouse
• Water plants with a watering can rather than a hose. Direct water to the base of
  each plant. It is better to give them a good soak every few days than a little bit of
  water every day.
• Use a dedicating drip-watering system or soaker hose/ perforated pipe to water plants. These can be attached to a timer on the tap that will deliver water at regular
  intervals throughout the day and week.
• Do not water lawns. Even if lawns go brown, they will recover immediately after
  rainfall
• Only use sprinklers on new lawns that are trying to put down roots
• Move pots to a shadier part of the garden

Always water early in the morning or late in the evening when evaporation is minimal

Others things to try:
• Water retention crystals or hydrogels can be used, especially in pots, to help keep moisture in the soil and avoid plant roots from dying out.
• Applying a good layer of bark chippings or gravel on borders helps to keep moisture in the soil. Once the garden is watered, the sun can only dry the top few inches of bark/gravel and will leave the underlying soil moist
• Replace dead plants with more drought- tolerant plants such as Abelia xgrandiflora, Acanthus spinosus, BerberisDarwinii, Buxus sempervirens, Ceanothus, Cistus x Purpureus, Colchicum autumnale, Euphorbia characias wulfenii, Sedum spectabile