Well you could start here:
http://www.basement.org.uk/
But I would start by having a discussion with any company about building regulations first off. That'll usually get the cowboys running away or making vague statements like "that won't be a problem".
In fact building regulations are often a major problem with cellar conversions due to the need to provide adequate ventillation and and meet fire regulations
Specifically you will be making the basement a habitable room which means that you either need to provide an external door or window for an escape or a protected stairway leading from the basement to a final exit.
In practice this building regulation often puts paid to plans for cellar conversion.
My advice is to think about being downstairs in the cellar with your son and finding that a fire has started in the kitchen. If you're happy that your conversion could handle this then fine - otherwise think again