The secret to growing lettuces that are crunchy but not tough or bitter is to grow them quickly. This means they need lots of water, well-drained beds, and warm rich soil.
Mulch will keep the moisture in the soil and keep the seedlings warm on cold spring evenings.
As a leaf vegetable, lettuces need nitrogen so plant them after your peas or broad beans have finished because their roots will have added nitrogen to the soil. Scatter compost, blood and bone or sheep pellets around the lettuce and add a little lime, especially after heavy rain. In summer they like shade from other vegetables so grow taller things next to them on their northern side.
Plant a few lettuces each week from now so they mature at different times. Select different coloured leaves to create patterns in your garden.
Choose different types of lettuce for different reasons. A leafy variety, rather than a hearting one, is great for pulling a few leaves off when you want to make a sandwich.
The plant will then grow more leaves. You can get a heritage lettuce mix seed packet from Koanga or for crunch try Webb's Wonderful, their hearting variety, a precursor to Iceberg.