Sorry to jump this thread, and I can not guide the OP to a respectable architect, but speaking as one I think it would be a very very wise choice! Yes it costs a bit more when you start out but it saves you a lot in the process
To daydream, whom I quoted here, I believe you've got this all wrong.
Do you know the saying "The first house you build alone, sell it, the second, rent it, the third you get to keep!"?
Architects are not just luxury mechanics, or painters or just designers for that matter. We are first and foremost sociologists. We are trained to think like you and analyze your every day lives and turn your space into more functional and comfortable than any builder would. Most builders can't interpret visions, they follow instructions, they won't be able to spot a problem before it occurs. The first thing I ever ask of a client is to describe to me their every day habits, and I can translate those habits into what would be comfortable for them! We also know how to guide you and make your dream better functional if we believe that there will be some problem with air and lighting for example, or proper circulation around the space. We don't change it, neither do we oversee it. On the contrary, we are there to make it true in every little detail.
A designer would basically help you decorate it and some enlightened ones would even be able to help you just as I could, with the synthesis or the layout of the space, but those are the excepted few and not the majority. They too can guide you better to realize your dream about how you want it to look, if you can't do it yourself. But an architect would help you to make it properly functional, while keeping your dream alive and enhancing it at the same time.
And yes there are people who know exactly what they want and how they want it like you and that's good both for them and me because they make my life easier and we can guide each other more easily (they to what they imagine and me to how that would be better realized) but that's not everyone.
To the OP again I can find a few names if you are interested. I have a few friends and colleagues there. Where in London are you?