Since you're all presumably in the NHS for your placement didn't you all have to sign something like doctors and nurses do, even as students, about professional conduct in the workplace and all? Didn't you all get a copy of various policies including bullying and harassment at least be made aware of where they are on the intranet? And don't radiographers have a code of conduct?
I'm guessing she didn't encounter many nurses yet as many of them would soon sit on behaviour like that.
And if the terrible twosome are behaving this way towards you what on earth is their attitude to patients like? Equally sneering? Not all the patients they're dealing with are going to be perfect 20 something specimens like they are who evidently (think they) understand things at a snap of the fingers, or hasn't that sunk in yet? I'd be inclined to say something along the lines of, thank you for your valued feedback, it's so good to know you know everything there is to know about your chosen profession already including appropriate standards of professional conduct towards other colleagues. I'm sure your patients will be equally reassured by your approach. Followed by the sweetest smile you can myster while looking daggers at them over your teacup.
And don't eat lunch with them, grit your teeth and have a conspicuously marked diary or calendar in red with the date when you wont have to put up with their sniping any longer that you leave open when they're around. And for God's sake stop helping this little snip with things like finding parking spaces and make her do her own work there as well or she'll continue to see you as a pushover instead of the kind and friendly person someone more mature would see. Sounds like taking the bus might do her good anyway, suspect she's spent her entire life getting others to do things for her.
How many more of these placements do you have to get through and is she likely to be on them?
You mention you're struggling, what are you finding difficult? Could you go and talk to your mentor about that in the first instance and just ask for clarification on the professional standards expected in the workplace at the same time? Good luck with the rest of your course and good on you for retraining when you're a bit older.