Alternatively, and more cheaply, you can learn to live with downtube shifters. I ran an 8 speed setup on my last commuter bike using a downtube shifter. I liked it. The only problem I had with it is I got so used to it I was always hunting around for the gear lever on the downtube of my other bikes.
I'd say get the Holdsworth and see how it goes with the downtube shifters. That would be a seriously good £100 worth and a properly nice ride. And you'll be surprised how quickly the shifters become absolutely second nature. Basically, going STI is going to involve a lot of cost and hassle. Is it really worth it? I'd say no.
Thanks all, this is exactly the kind of info. I was looking for.
I know what you're saying about the Trek bike. It looks good. (Too big for me but I'm sure there are more similar ones out there.) I've just got an idea that I would like a bike with a bit more character, and it would be nice to customise something. I prefer the look of thin tube steel bikes too. I'm looking at a Holdsworth 531 12 speed for £100 - maybe I'm crazy and that's not such good value? By the time I got a new cassette, back wheel, chain, STI levers etc., I wonder how much I would have spent? (any ideas?) And a cold set would probably be required too...