It is available in the United Kingdom. Valbazen, available here in the US is one of the best wormers. It is given orally 1/2 ml per chicken once and repeated in 10 days. Flubenvet is given in the feed over 5-6 days, and an exact dosage is hard to gage. If you are looking for something to treat a lot of chickens at once, there is Wormout Gel put in the water for 2 straight days. It gets all chicken worms as Valbazen does, and is available online.
I prefer Valbazen because you dose each chicken by mouth with an accurate amount. You give a 1/2 ml dose to kill the worms, then repeat it in 10-14 days to get any worm eggs that hatch. For bantams, I would reduce the dosage to 1/4 ml,mand for very large birds or roosters, 3/4 ml may be needed.