Architect.

While I'm not sure how the British system works, in the states you'd work with an architect and then a general contractor - or a capable design-build company, and a kitchen-bath professional as well. You'll need creative design input along with experienced competence in structure, plumbing, electrical, etc. Seems important that you do a good deal of planning and budgeting before you start work. Since these various projects encompass most of the house, they are all inter-related.

By the way, as kitchen-bath pro's, we prefer to be involved early in the concept planning stages of projects like this. Good Luck!

You will get more design ideas and economic values returned from working with an architect.

Find at least two well recommended architects to interview in your area. Talk to their clients and ask about the process and how much they liked working with their architect. They should have glowing reviews. Look at several of each architect's projects. Would the client hire that same architect again if they were to start over?

Choose the architect based on the recommendations and your comfort with the architect.

You should get guidance through the entire design and building process by your architect plus the final goal is for you to have a home that is a delight to live in..

Architect is a must.

I agree with the previous posters - start with an Architect who will give you the drawings to get approval with, and for your builders to create the shell etc - but also add to the mix someone, to help you design the bedroom, bathroom and new kitchen at an early stage - so that windows can be sized to fit in with the layout as well as the general structure of the building etc - a concept planner as it were ! Then, with their drawings you can shop around for the best deals and compare like for like !
If you are really lucky - you will find an architect that does all of the above - if not then someone like me, can help with the concept planning side - click on my name to go to my pro page to see the sort of drawings and designs I can offer ?!
I'm based in UK and all work done via email !
hope that helps
Karen at OnePlan :-)
http://www.houzz.com/pro/oneplan/oneplan

I believe that OnePlan could do all of these things for you. I highly recommend her.

Use either an architect who specializes in designing homes or a Residential Designer. There are many draftsmen who are well versed in residential. Discuss their background on the phone. Be clear about your buget before you start.