Florence Fennel is a tricky crop to grow. Although it looks beautiful on any plot and tastes equally special, it isn't ideally suited to growing in the UK and as a consequence will bolt at the drop of a hat as soon as conditions turn against it.
The first thing to consider is that Fennel is very sensitive to shortening days so I tend to plant mine after midsummer to aim for an autumn crop. I start them off two seeds to a pot in the green house sometime around mid / end of June. Once the seedlings come up I let them grow on for a week or two until they are big enough to identify the strongest and thin accordingly. As far as planting out is concerned, slugs love the rich, young seedlings so I wait until they get to a decent size before moving them outside. If they are maybe 8 - 10cm in height they will have the best chance possible of outgrowing the jaws of our slimy friends.
Fennel likes to be planted in well drained, rich soil and they also need to be watered thoroughly as they grow. This is not a crop that can miss a day's watering in hot weather! Mulching around the plants can also help to preserve moisture and is well worth the extra effort. When it gets to about September some people earth up the developing bulb but I normally forget to do this and have never had any problems as a result. By early October you should have a bed full of juicy bulbs ready to be picked. Unless it's an exceptionally hot year we will never get big fat bulbs like the ones grown in hotter climes but you can get a perfectly satisfactory crop that looks and tastes gorgeous.
The plants will not tolerate any frost so all need to be picked before the weather gets too cold. They also don't store brilliantly so are definitely best eaten fresh. And if they do go to seed don't despair - the flowers can be a dramatic and beautiful addition to your plot.