When people say plans, they mean something an architect draws - not something a builder draws on the pack of an envelope
Plans are important - they let you show building control how it was meant to be built (and that it was designed to meet regs) and they let you and the builder have an agreed plan to build against so you both know what you are getting!
Try this guy - he drew plans for a garage (that I couldn't afford to have built, sadly) and was very reasonable - he can deal with planning control & building control for you, too:
https://www.davidtaylordesignservice.co.uk/(FWIW it was £800 in 2011 to have plans drawn including the cost of submitting to building control, local authority and planning application, and that included £80 for structural engineers calcs for beam loading - I paid around half that, because we never submitted for building control/LA etc, in the end)