If your limit for a cooked breakfast and a hot drink, is £6, then you are going to have very limited options on the A303. I guess there might be a van in a layby somewhere.
Can you clarify, whether you are looking for the 'best brekkie' or the cheapest ('VFM')?
My Dad has to trek the A303 a lot for work, and swears by this place:
http://www.mattiadiner.co.uk/
http://www.mattiadiner.co.uk/docs/Mattia-CombinedBreakfastMenu.pdf