Can't find my growing instructions at the moment, but they need a long time to grow then mature before you can harvest them as luffas. They're easy to grow but once they mature, they need to be left on the vine in a sheltered area for about 2-3 more months to 'cure'. Only then can you peel and clean them up for use.
A few tips, first soak the seeds in water for 24 hours before planting one per sizeable pot into moist soil. And in the UK, you'll need to start the seeds a month or two before the main growing season so they're already growing. Like all gourds and cucurbits, they detest having their roots disturbed so make sure the pot is fairly large because they're fast growers. The contents can later be transplanted to the garden.
They're a climbing vine so you'll need strong support for them to dangle their fruit and remain straight (there's no untwisting a crooked luffah). They're a heat loving plant that needs full sun, so a greenhouse or sunny sheltered area are perfect.
Once bad weather appears in fall, you can bring in the luffa to finish curing but do leave a good chunk of vine attached and let this hang somewhere sheltered and dry (shed would be good).
here's more info:
http://www.luffa.info/luffagrowing.htm