Before anything else if you've not done so already I'd measure the height you have from floor to ridge. Anything less than about 2600 and the project is probably a no go anyway (typically you need about 250mm for the floor and 125 at rafter level) so 2600 - 250 - 125 = 2225. Just about OK for a floor - ceiling though you may need dormers if the ridge is quite low too. Though you may get the floor lower than that depending on the design. Getting the stairs in can also be fiddly.
It sounds like you're happy to go down this route anyway but unless you are prepared to put the time and effort in to learn about the required Building Regulations and construction techniques then it would be prudent to employ someone to do you a set of Building Regs drawings. People on here (DIYnot) do do loft conversions without any drawings (apart from structural engineers which is a must no matter what) but they do seem to come a cropper every now and then (though having a set of drawings is not a guarantee to avoid odd things cropping up either).
If good (ie not just a set of notes) then these drawings will guide you through the process providing you with a specification and demonstrate what you need to do to achieve your conversion. These drawings are submitted to Building Control and once approved you or your builder can start building away confident that if you adhere to their requirements you shouldn't get too many hiccups.
Don't waste your money with architects, generally speaking they are overkill for a lofty especially any commercial practices ie only employ one man bands otherwise you'll be paying £80+ an hour. So, employing a one man band architectural technician would be prudent, any respectable ones will give you the first hour or so to come round, have a look and a chat and get an idea of what you want to do and then write to you with a formal quote. Explain that you want drawings that a builder can actually build from (ie not just a plan and a set of notes cut and pasted from the last job) and ask to see a set of their Building Regs drawings. Don't be afraid to ask for references if you like. A good source of finding popular technicians can be looking at your local authority planning applications website and seeing who the popular technicians are. Ask for a fixed price (unless you are not sure what you want and want some feasibility done).
With regards to your structural engineer, (calculations are required for Building Control) a good technician will know pretty much what their usual engineers require and will integrate this into the drawings. How you employ a structural engineer is up to you/your technician. You will find some technicians charge you an overall price to include the SE calcs, others will expect you to employ and deal with the SE on your own and various other scenarios. Personally, with regards to SE's, within my fixed fees I allow for arranging quotes from, liaising with and integrating their final design within my drawings although the SE charges the client direct.
Personally, in my opinion, you should be wary of any builder prepared to give you anything but a rough estimate without a set of decent drawings.
More here on Planning and Regs:
http://www.planningportal.gov.uk/permission/commonprojects/loftconversion/
