Although my business sells promotional products, I'm actually a qualified chartered accountant. Unfortunately(well to be honest, very fortunately!) I didn't work in tax so I don't know the exact ins and outs but hopefully I can give you a rough idea:
First of all I assume you are a sole trader and complete an annual self assessment tax return? As opposed to being a Ltd company?
Cars: First you need to work out the percentage business use of your car. I think you have to do this with a decent level of estimation. For eg by noting your mileage at the start of the year on your car and noting it at the end. Then subtracting mileage to and from work. Once you have figured out how many business miles you do, there are 2 ways to work out your expenses for the year
1) You can claim 45p per mile for the first 10,000 miles of business travel and 25p per mile thereafter. The 45p per mile is meant to be an all inclusive cost which covers petrol, car repairs and MOT throughout the year etc.
2)'The full cost method' which is basically your exact car expenditure in the year using receipts to calculate mileage and repair costs
Phone: For the phone you are meant to claim only the business expenses. Best way to do it would be to take a months statement and make a rough guess of the percentage cost of calls that are business related
Broadband: Similar to above, keep a log of how many hours you use broadband for work and use that ratio against the total broadband costs to come to your broadband allowable expenses figure
There's quite a few other things you can claim too, especially if you're working at home, for e.g a percentage of your household expenses.
Let me know if you have any other questions and I'll try to see if I can help
*Edit* This link may be of use
http://www.hmrc.gov.uk/helpsheets/hs222.pdf#page=3 Page 6 - shows things you can expense and page 8 shows the car allowances discussed above