I'm sure what you are referring to are expenses that you can offset against your income. These reduce your net income (on which you pay tax). There are no 'basic allowances' as such.
What you should be ensuring you show fully as 'business expenses' are costs for tools specific for your job, the mileage at 40p per mile for travelling to customers' sites (up to 10000 miles in the tax year - less after that), plus anything else you can reasonably think of that you truly cost you.
Actually the 40p per mile thing would only be relevant if he uses one vehicle for both business and personal mileage and cannot tell apart the business costs from the personal ones.
If he for instance has a van for the carpentry business and a car for his own personal use and never the twain shall meet then he should simply claim the costs of running the van in full, whether that works out at more or less than 40p per mile.
How right you are, S. That's the difference between the knowledge of a guy who does this for a living and one that merely looks after his own interests in these matters.