Travel to clients - the 24 month rule and the 40% rule
Commuting is not allowable for tax. HMRC have decided at what point regular travelling to a client becomes a commute.
If you sign a contract of two years or more with a client, travel and subsistence in connection with visiting the client is not allowable (unless you visit a number of different locations belonging to the client on an irregular basis)
The same applies if you enter into a continuous series of contracts with a client - at the point that one of those contracts commits you to a period of time that overall takes you over 24 months you must cease claiming travel and subsistence.
In addition, to be eligible for tax relief no client should take more than 40% of your charging time whilst you are employed by your limited company.