In terms of HMRC I suggest that people read their webs pages not the potted versions here.
A few facts as self employed you can claim 40p a mile for work travel (or?45p if two people are in a car) or the actual running costs upto 10000 miles per year.
Travel costs are a legitimate business expense for any business and therefore form part of the profit/loss they are not offset against tax as such.
For an employee the situation is explained as
http://www.hmrc.gov.uk/mileage/employee-factsheet.htm
In short you can be paid upto 40p per mile without paying tax on the amount. Work journeys are closely defined.
For site work of less than 2 years duration see
http://www.hmrc.gov.uk/helpsheets/490-chapter3.pdf
A workplace is a place where the employeeâs attendance is necessary for the performance of the duties of that employment.
3.3 So, in general, there is no relief for the cost of travel between an employeeâs permanent.
"A period of attendance at a place is not regarded as of limited duration or for a temporary purpose if it is all or almost all of the period for which the employee is likely to hold, or continue to hold, the employment. Example Everton is taken on for a fixed term employment of 18 months to work at a particular site. No relief is available for the cost of travel to and from the site during
that period."
I note that these are rules on tax not on what the employer should pay. So I am not obliged to pay 40p a mile just because this is the revenue maximum. Similarly I am not obliged to pay UKP 175 a head for a Christmas party. An employer can choose to pay to travel expenses which are outside of the revenue rules but that is a benefit in kind and the employee is liable to pay tax on the value (40p per mile?) and the employer NIC
The same is true of subsistence payment which are taxable
http://www.hmrc.gov.uk/helpsheets/490-chapter9.pdf although the employee can claim tax relief on actual expenses.