1. First and foremost, water as much as you can before you leave. Give everything a long deep soak. But to make that watering last here are some tips:

Mulch the soil after you've watered. Grass cuttings, cardboard, leafmould, compost, a thick layer of wet newspaper weighted down, will all provide cover to help keep the soil moist underneath. They'll also suppress the weeds.
Create some shade above the plant. Use fleece, netting, cardboard boxes or similar, which will protect plants such as lettuces from the burning sun. Can you shade the greenhouse or conservatory?
Either move pots to a shady area, or sink them into damp soil - again in the shade - up to the rim. Clay pots will absorb the soil's moisture; plastic pots will keep cooler than if left above ground.
Move hanging baskets out of direct sun, and put them in the shade with other pots.

2. Pick everything that is ripe, and process ready for storage. Some vegetables, such as tomatoes and beans can go in the fridge or freezer, to be cooked/eaten on return. Others such as potatoes, courgettes and onions can be stored in a cool dark place.

3. Ask a neighbour or friend to help with watering. Tell them which plants are most important to you, and how often they need a soak. To help them, gather any pots or hanging baskets together, and put watering cans near to butts, tanks or outside tap. The offer to help themselves to any surplus fruit and veg is sure to be appreciated!

4. Pick any flowers to encourage new blooms. Sweet peas and roses would make a lovely bunch for your helpful watering neighbour. Cut back others which are nearly over, such as geraniums, delphiniums and alchemilla mollis. They might have a second flowering in September.

5. Finally, if you have time, cut the grass and weed. It will make your garden so much nicer to come home to.

Happy holidays!