Simply in terms of cold, hard economics Saffron has to be the singularly most worthwhile crop that can be grown by UK gardeners. For by planting up a bed the size of the average dining table just once, you will get a constant supply of the world most expensive spice every year for at least the next 10 years, producing more than the average family can eat. Compare that with onions, which you need to labouriously sow and plant every year, are incredibly cheap to buy and widely available & you can see why I feel there simply is no contest.
The best time to start your own mini Saffron plantation is in late summer, planting the corms about 10cm apart in a hole 10cm deep, then watering them in really well. They love sunny, well drained sites that mimic their Mediterrenean home, so help them along by digging in a little grit into each hole before planting. You can even plant these amoungst grass, just like the traditional "park lawn" crocus, although if you do this you should be careful NOT to mow when the leaves are out, from September until April. For lazy gardeners like me however that does give you a fantastic excuse for a slightly wayward-looking lawn.
When buying your crocus corms there are two golden rules. Make sure you pick the largest corms you can, as these will flower far earlier. About 2.5cm across is the best size to go for. The most important rule of all however is to make sure the corms you buy are definately the edible saffron crocus, these will be clearly labelled "Crocus sativus". This is because there is a similar-looking autumn-flowering plant that is toxic, and confusingly goes by the name "Meadow Saffron". If in any doubt, buy the corms from a special "grow your own" range, (see supplier link on the left) where you will be guaranteed of no botanical mix ups.