If you contract through your own personal service company (PSC), you will be an employee of that company and you have to obey the strict tax rules that apply to employees' travel deductions when claiming expenses from your PSC.
The first rule is that the cost of ordinary commuting cannot be claimed. This is defined as travel to a permanent workplace, which is somewhere attended regularly to perform the duties of the employment.Travel costs to a temporary workplace can be claimed, but the conditions that make a workplace 'temporary' must be met.
A place is not a temporary workplace if the employee attends for a continuous period of more than 24 months, or the attendance is expected to last more than 24 months. If your PSC takes on a contract that is expected to last say 36 months at one location, you can't claim travel costs to that location, as your workplace is not a temporary workplace from the start of the contract.
Another definition of 'temporary workplace' is one which the worker attends to perform a task of limited duration or for some other temporary purpose. HMRC has a rule of thumb that if the worker is attending a place for 40% or more of his working time, that is a permanent workplace and travel costs to the location can't be claimed.
If you work at your client's office for say 15 hours per week out of a 40 hour normal working week, your client's office is a temporary location even if the contract exceeds 24 months. Please discuss the matter of travel expenses with us before you take on a long contract, as the deductibility of the travel costs may tip the balance on whether the contract is worthwhile.