I'm really surprised that people are talking about spending £3-4k on a road bike for TTs. Why on earth would you do that? For that money you can get a road bike and a TT bike! Granted they'll both be cheaper bikes, but much more important than having DA over 105 is the fact that you'll be able to set each one up for its specific discipline. A tri-bar position is different from a roadie position: the reach is shorter, the saddle is further forward and higher up, the bars are lower. The optimal saddle will also vary from discipline to discipline: one that is comfy when you're sitting up on your road bike might be agony when you're crouched over the nose in a TT.
I've had one bike do both jobs for the past few years, due to money and space constraints. I managed to get a TT position by having some very deep drop bars so I could run them with a shorter stem and still have the hoods in the right place. I also had two saddles on separate seatposts, which I swapped out depending on what I wanted to use the bike for, at the same time as I dropped the handlebars. All this was a bit of a faff, and I still couldn't get the bars as low as I wanted because of my bike's 16 cm headtube, which is about 6 cm taller than on the TT frame I've just bought and am building up.
If you get one bike, it'll be a compromise whatever you do. If you want to TT, get a TT bike. If you want to do both disciplines and have the space and money for two bikes, do that.
