I suggest £1000 becauase if the bug bites (which most likely it will) then it will save you £s upgrading.
But the great thing about the road, is that unless you race seriously, there is no real need to upgrade. Once you get a basically okay bike with tiagra (so usable shifters), sort of thing you can pick up 2nd hand for £450 or so, all upgrading does is potentially make you slightly faster. It isn't like mountain biking where fancy parts can make a big difference to how possible it is to ride, due to breaking easily or whatever.
Yeah, sometimes I get dropped by people on £4000 super bikes, and maybe they are slightly faster, but to be honest, I've never felt that it was anything to do with what they were riding, I've overtaken people with much fancier bikes than mine, and been overtaken by some bugger on a fixy that couldn't have been worth more than £20.
I also did a good few miles on a 500NZD (about £150) 2nd hand road bike in New Zealand, and as long as it was maintained (oiled every so often), it was fine. The only thing that annoyed me was the funny Sora shifters. Cheap road bikes have got surprisingly good. You kind of notice the difference in a fancier bike, but really it isn't anywhere near as big as you'd think.
Joe