the
halfords carrera range is not too bad at all for the money. the pitfalls lie in that they can come poorly set up which, assuming you're a newb to the whole thing, can go undetected and result in problems sooner ot later.
i went down the halfords route, got a
carrera virtuoso for £250 and have since spent at least as much again upgrading it, but it's now as good as a bike you'd pay £600 for anyway. (so if i'd bought it, found that a road bike was not for me, i'd only have wasted £250 less what i could have sold it on for). you can upgrade as you go, which if your budget is limited is very useful. don't listen to people who tell you you need to spend a grand on a decent bike, they're just trying to show off.
if you have a good local bike shop, see what they have. they may be able to do a second hand steel frame with "modern" things like combined gear/brake levers which are a lot more user friendly imho than downtube shifters.
i would avoid
ebay unless you feel you know enough about bikes in general, especially as you've no way of knowing if a bike is stolen. if the seller is local, ask to see it and take someone that knows about bikes with you.
if your local cycling club has a website, you may find thay have a classified section and be able to pick up a bargain.
good luck