Once you're over 16 years of age you can sign the consent form to have an abortion. The health professionals would have to be sure that you were able to make the decision and understand the consequences and risks of the procedure.
In the case of a serious underlying heart condition, it is likely that you would have to stay in the hospital overnight to be monitored, and I think that the health professionals would be happier if your mum or a responsible adult knew about your situation.
I know that it is difficult to tell your mum about your pregnancy, but it is also going to be a very hard secret to keep from her for the rest of your life. Is there anyone who can help you to tell her? Do you have anyone else close to you who can help you and support you? If you would like confidential help, have a look for your nearest unplanned pregnancy advice