In Roman times parsnip was considered an aphrodisiac. Have you experienced any "symptoms"?
Loosen the soil to about 40cm to ensure the roots can grow down easily. Seeds must be "fresh" or only from last season as they lose their germination capacity quickly.
Sow in trenches about 2cm deep. Place one seed every 15cm. The roots do not take up much space but the leaves can grow horizontally.
Germination can be slow if it is cold or wet. Be patient. But for this reason wait until September.
SEEDS vs SEEDLINGS
Roots always do better planted as seeds in situ than seedlings. Parsnip is frost tolerant so you can plant now.
I haven't come across any varieties with short enough roots to do well in containers. But in saying that, if you have a deep enough pot you can grow any root vegetables.
Parsnip enjoy the companion of lettuce and peas but ensure they do not become shaded by these crops. Lettuces and other fast growing crops like carrots grow well between rows while parsnips are becoming established.
PESTS AND DISEASES
Unfortunately, if you get parsnip canker (a brown or red fungal disease) you will need to remove and dispose of your parsnips. Do not compost.
Parsnips take between 4 - 6 months to mature depending on what season you plant them in. Therefore they may not be worth the space that they take to grow.