Try searching for an architectural designer for your local area, should be much cheaper than a RIBA architect. Despite the modest size I recommend a building regulation ‘full plans’ package so you can obtain accurate quotations from builders, the plans will then be checked by Building Control prior to site commencement which provides the opportunity to raise any queries before you start building. Depending on the design of the extension and modifications to your existing house you may also require a structural engineer to provide calculations for steel work, for example if you want to open a room into another etc.
FYI I’m a self employed architectural designer located in Somerset.