I asked a similar question a while ago and was informed by the tax office
that a car bought to use as a hackney was still classed as a car and subject to a limit of £3000 per year but a purpose built taxi such as an E7 or a TX4 was classed as a commercial vehicle and should be claimed under the new Annual Investment Allowance
Eg Profit after allowable expenses = 15,000
Annual Investment Allowance 100% - 27500 (cost of new E7)
Loss for year to be carried over to next year -12500
(Reduce Annual Investment Allowance by percentage of personal usage before application.)
I was also told that interest on loans and HP were not to be included in the AIA but claimed in the allowable expenses section.
More detailed explanation can be found in this older post.
http://taxi-driver.co.uk/phpBB2/viewtopic.php?t=10906