You can change any part of your name at any time, whether you are married or divorced.
If you separate, you can revert back to using your maiden name (or a completely different name) immediately by using a deed poll.
It is a good way of announcing that you are no longer together and that you are an independent person again without waiting for a divorce to complete.
You need to notify your husband of your change of surname if proceedings have not started so that the documentation is in the correct names. Changing your surname doesn't affect divorce proceedings or your eligibility to be divorced.
Using a new title
You can use any title you wish. You might like to be called 'Mrs.' even after divorce, or you may prefer 'Ms' or 'Miss'. If you don't change your surname, you don't need to complete any legal documentation to change your title - just start using it. If you do alter it by deed poll, then you can specify your new title in that document.
On divorce, a woman may revert to using her maiden name either using a deed poll, or by using her final order and her marriage certificate. Since some companies do not accept the latter as proof, the best way to ensure a surname is recognised is to use a deed poll (which all UK organisations will recognise). A deed poll also has the advantage that other names can be changed at the same time or a completely new surname could be chosen.
Note that if you change your title from 'Mrs' to 'Miss' or 'Ms' before the divorce has been finalised, you will still have to disclose that your marital status is 'married' on any application forms, including those for financial products (such as loans or insurance). Once the divorce is finalised, you can declare yourself as 'single' again.
If you have children, then, with your (ex) husband's permission, and that of anybody else with parental responsibility for your children, you can change their surnames to your new one or to any other (such as a double barrelled one). If you are the main custodian of your children, then their having the same surname as you helps reduce explanation of your marital situation.
On being widowed
A widow's husband's death certificate and her marriage certificate (showing her maiden name) are all that are usually required to prove to government departments that she has changed her name back to her maiden name on her husband's death.
However, some institutions, notably banks, may not accept these documents. If so, a deed poll can be used as it is recognised as legally valid by all UK organisations.
If the widow wants to change her name completely (such as to another family name), she needs to do so by deed poll.
Further information and useful documents
You may be interested in our free deed poll template that allows you to change your name without involving any third party solicitors or companies.