Try your local trout fishery and see if they do lessons. I went along to a group lesson at Carsington in Derbyshire and thoroughly enjoyed it. There were about eight of us and we got an hour of explanation of the basics, an hour and a half of actual casting tuition followed by an hour of basic fly selection, tactics and knots for those who needed the very basics. Topped off with an afternoon let loose on the water we all had a great time.
If you can find something similar locally I would recommend the group lesson, I didn't feel pressured and could have a bit of a rest when my arm got tired - which it will until you get used to casting.