Generally it's only legal (and more importantly, admissible as evidence in court) if all participants are aware of the recording and consent to it. So you will need to say clearly at the start of the call "I would like to record this call, are you happy for me to do so?" and then if they say yes you can start the recording. If they say no then you shouldn't.
As noted if you can't trust your GP you should just find another GP. That said, GPs do not generally act maliciously (in fact, non-malfeasance is a cornerstone of medical ethics). So you may want to consider whether the reason she put that in your records was because she believed it was in your best interests to have it noted.
I'd also point out that a GP has a lot more to lose than you do for putting something "malicious" in your medical records, so even if they were unethical, it would not make any sense practically for them to do so - it's a much bigger problem for them if they get referred to the GMC or investigated by the police (even if they aren't indicted in either case). So again - is it really because they were being malicious, or was it because you disagreed with their clinical reasoning and are unhappy with the outcome?
You may want to actually talk to your GP about your feelings regarding the relationship and why you feel this way, as she may be able to explain exactly why she did what you purported (or even if she did - you may have the wrong end of the stick and nothing was ever noted on your medical record). I very strongly suspect the reality of the situation may be a bit different to what you are thinking it is, and talking that through may be more productive than attempting to launch legal action against her (as if it ends up being baseless and/or the court sides with her, you could end up in a very bad position financially, to say the least).