A lot of solicitors have non-law degrees and, indeed, I'm led to believe that a large number of those read history as undergraduates.
If you do mention it in your personal statement, make sure you have several good reasons why you're not applying for law! Try to make some good links between the skills a history degree will equip you with and those needed to be a lawyer. It might be a bit obvious to say because you're interested in 'legal history' but I see no harm in saying that work experience with a solicitor has led you to consider the possibilities of a legal career in the future. The university will probably view quite favourably the fact you've done some work experience (if, indeed, you have) as it demonstrates initiative, a commitment to finding out about things, etc.
It may even be an asset to express a broad range of interests but, of course, you want to stress that, while these are important, they are secondary to your interest in studying history.
Personally, I see nothing wrong with mentioning future career aspirations so long as you relate them to the course you're applying for. You don't want to apply for history then say you want to be a solicitor without qualifying it in some way. There's a chance that they'll just think you're applying for history under the (false?) impression that it's less competitive.
A sentence will probably suffice in incorporating your interest but make sure it doesn't dominate your personal statement or call your passion for your degree subject into question.