Hello Neil, ive just posted a similar thread. In a similar dilema (entered IM Wales). Did anybody get back to you? Im thinking of a Boardman Air or Giant TCR. Thanks.
This is a difficult one. There are plenty of superb bikes available for £1,500 or less, but it very much depends on what you're looking for specifically.
Personally, I ride a Cannondale CAAD10 and I love it to bits. It's a fantastic bike and you can get one for well under your price limit (http://www.evanscycles.com/products/cannondale/caad10-5-105-2013-compact-road-bike-ec042916#select). But my best advice is, if you can, try a few different bikes. Talk to some experts about fit, about what you're looking for in a ride and about your goals. If you're going to be racing 180km on it, you need to make sure you get something that won't cripple you in the process!
Unless you're an absolutely top cyclist, what matters most is the position you can hold on the bike, rather than the bike itself. The rider accounts for far, far more drag than the bike so if you can find something on which you can hold a good aero position (and make sure you can still run afterwards), that's what you need to aim for.