Donor bikes (broken frames or frames of sizes that don't fit but have the parts you want that you get a great deal on) are the way to go. Also, the UK shops for groupsets. Sounds like you don't know what size frame to buy, so maybe this is more than you want to bite off as a first timer.
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Bikes: 1996 Eddy Merckx Titanium EX, 1989/90 Colnago Super(issimo?) Piu(?), 1990 Concorde Aquila(hit by car while riding), others in build queue "when I get the time"