A recent tax case has been causing a degree of disquiet amongst the self-employed (that's to say, sole traders and partners - those trading via a limited company fall under an entirely different set of rules), specifically those with significant travel expenses.
Dr Samadian was (presumably still is!) an employed NHS doctor, who also did work a couple of days a week at two private hospitals, on a regular and predictable basis. He had a home office, which he claimed as his main place of work as a sole trader. On his tax return each year he and his accountants claimed the costs of travelling from the main place of work (the home office) to those two private hospitals as a cost, reducing Dr Samadian's profits and therefore tax bill. Most accountants would have done the same.
However, HMRC disagreed. They thought that the two private hospitals should be considered as two more main places of work - Dr Samadian went there on an utterly predictable basis, the same day each week for each hospital, and he never actually saw any of the patients concerned at his home office. They didn't think that the travel expenses should be used to reduce profits and tax; they were just commuting expenses and therefore the doctor's own problem.
When the case went to a tax tribunal, HMRC won, and Dr Samadian was told he can't claim the travel costs. The regular pattern of the travel means that he's just going to a regular place of work. Having established that principle, HMRC are quite likely to want to use it in future. Any sole traders or partners who travel to sites on a regular basis ought at least to be aware of the implications and think about how their working patterns might sit with this interpretation of the rules. If you might be in this spot, one way that you can look at it is to say - would a friend or family member know where to get hold of you on a particular day of the week? If so, then even if you've a main workplace elsewhere, then it no longer necessarily follows that your travel expenses to wherever you are on that day are going to save you tax.
If in doubt, have a chat to your accountant!