How do I purchase a drop bar bike that will be converted to flat bar?
Converting between drop and flat bars is generally a huge amount of hassle. As you say, there are all kinds of incompatibility issues around brakes and shifters, and the geometry of the frame is designed with particular bars in mind, because changing the bars makes a big difference to riding position.  There's no point spending £1400 on a bike and then spending a couple of hundred more to turn it into some weird Frankenbike that will ride  worse than a hybrid costing a quarter of the price.

I wouldn't advise spending anything like that much money on a bike that you're not sure you'll be able to ride comfortably. Decathlon will sell you an entry-level road bike for £250; Halfords will sell you one for £180. You can probably get an old, second-hand one for £50&nbsp;&ndash; and then sell it again for the same price.  Even if you get an old bike with down-tube shifters, you can still evaluate the riding position. Or hire a bike for a weekend or a week.  Do you know somebody who can lend you one?

Once you know what you want, you can consider more expensive options.