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.