I run a business and want to know:

What is the difference “Tax Wise” between buying and leasing a car?

When a business buys capital equipment the items are put in a Capital Allowance Pool.
The amount of tax relief taken on a car as part of this pool is geared to the C02 emission of the vehicle.
In short it works like this:
If the car has a C02 rating of 0-75g*/km then 100% 1st Year Allowance applies
76 -130** g/km 18% Pool
131or more g/km = 8% Pool
*Moving ton 50g/km from April 2018
**110g/km from April 2018
If you contract hire
0-130***g/km = 100% of rental allowable
131+g/km =85% of rentals allowable
*** This INCLUDES any unrelieved VAT 
*** Moves to 110g/km from April 2018
Cars first available for use on or before the 31/03/2018, the old rules will apply for the duration of the lease (including extensions to that lease) If the car Has a C02 of more than 130g/km (110 g/km from April 2018) un-reclaimed VAT is also restricted to 85%.
International Accountancy Standards Board
From January 2019 
  • FRS 16 eliminates the classification of leases as either Operating or Finance Leases.
  • Instead all leases are treated in a similar way to finance lease applying IAS 17
  • Leases are “Capitalised” by recognising the present value of the lease rentals and showing them either as leased assets or together with property, plant and equipment.
LCV’s (Vans, Trucks and Pick-ups)
Purchased –can claim 100% write down allowance in year one via your annual investment allowance (subject to an overall limit of £200,000 per financial year)
Leased-claim 100% of the rental against your tax position providing no private use (Sole Traders and Partnership) or if you are a limited company.
100% of the VAT may also be reclaimed provided you are:
a) VAT registered 
b) Not on Flat Rate VAT scheme
c) No private use
d) No “Exempt Turnover”
If you provide your employees with a “Cash Allowance” to enable them to make their own arrangements be aware as their employer you are still liable for much of the responsibility in respect of the fitness of the vehicle, insurance and fitness of the driver to drive. Strongly suggest you check our sister site www.autoplanfleetmaster.co.uk.

I get a Company Car—how much tax will I pay?

The rate of tax you will pay on your company car is geared to your own tax rate and the C02 rating of the car you have chosen.
Below is a Benefit in Kind C02 Bands by Tax Year g/km to assist you.
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When taking a quotation for your company car the leasing company will advise you of the C02 rating of the vehicle.
I drive the company van how much tax do I pay?
Taxable benefit of £3150 per year 2015 /2016
Fuel benefit £594 per year 2015/2016
Moving 2016/7 to £3170
Fuel benefit going up 2016/7 to £598
Total £3768 means 
20% tax payer pays £754 a year and a 40% tax band pays £1507 per year.
In April 2017 the figure may rise by the Retail Price Index.