Lettuce Lollo Rosso
For salad leaves through the summer, lettuce should be your banker. Whilst it can attract a few slugs you'll have none of the hassle with flea beetles that get attracted to japanese greens like mizuna and mustard leaves (sow these from July onwards when flea beetles have disappeared).
Lettuce is also very well suited to the British climate. Conveniently it loves moisture and prefers a little shade. It will even keep on growing through the moderate frosts later in the year.
With this variety you can pull off individual outer leaves as you need them, and the plant will carry on growing.
Time from seed to plate: 10 weeks
Sowing Calendar
Sow
Lettuce seed can be sown directly into your soil, but I prefer to sow in gutters. It's a quick way to sow, and it's easy to tease out individual plants for planting out. Fill the gutter with compost, firm and create a shallow groove down the centre about 1/2 an inch deep. Sprinkle the seed in sparingly, aiming for one every 1/2in or so. Water gently but thoroughly.
Sowing method: Gutters
Grow
Transfer your seedlings to your plot when they're 2-3in tall. Separate the plants from one another in the gutter using a trowel and plant out individually, leaving 12in between plants in both directions. Water in and scatter with some organic slug killer till the plants establish themselves.
Removing any decaying leaves from the bottom of your plants as they grow will also help to deter slugs.
Harvest
You have two harvesting options: Pick the whole plant, or twist off individual leaves from the outside, while the plant carries on growing from the centre - creating a mini lettuce tree that provides a continuous supply of leaves.
Soak the leaves in water for five minutes after you've picked them for maximum crispness.