What does (did) your employment contract state? In the UK, it's pretty common that your employer takes ownership of any work that you carry out whilst under their employ. So if as part of your job (whether you were employed specifically as a web designer or not) you created/updated their website, then it's their site, and they own it. If they want to make further updates to it (that may include removing your name), then it's not rude, it's their right.
Ofcourse if you have a contract that lays out the details of the web development work you carried out for them, and part of that contract is that your name appear in the footer of the site, then they have breached that contract. A polite word in their ear, reminding them of what they agreed to, might do the trick.