In the UK (unlike in many other countries) anyone is free to use any name they like, at any time. (There are a few very limited exceptions, such as people on the sex offender's register who have to notify the police about any name changes. Also, of course, it's illegal to claim to be someone else in order to commit fraud, but such exceptions aren't relevant here).
So I can call myself John Smith on Monday. On Tuesday I can be Herbert Jones. On Wednesday I can be Martha Price. On even numbered hours on Thursday I can be Louise Humphrey Bloggs, but during the odd numbered hours I can be Horatio El Presidente. It's all perfectly legal and there's no need for any formal declarations of name changes (such as deed polls).
Of course 'officials' (such as banks, council tax officers, tax men, etc) want to be able to keep track of people. So it makes sense to limit the number of names that you use, but (as far as UK law is concerned) you've always been free to choose between your married and maiden names. (Many actresses, for example, will retain their maiden name for professional purposes but use their married name for other purposes. As long as they don't try to defraud the tax man by doing so, it's entirely legal).
You've posted this question under 'Law'. The simple 'legal' answer is that you need to do absolutely nothing to start using your maiden name again. In practice you might need to contact individual organisations (such as your bank or HMRC) with a written declaration stating that you'll be using your maiden name in future. (For example, if you used to sign your cheques as 'Sara Bloggs', but you'll now sign as 'Sara Jones', your bank will need a new specimen signature).
Chris