Is there some reason in particular you think you may have ARVD/C? Most patients (and even doctors!) have never even heard of this zebra diagnosis. You could still consider getting the cardiac MRI to rule out other etiologies, such as myocarditis, which is not well seen on echocardiography. I am not familiar with how the health system works in the UK. To alleviate your concerns, I guess you could pay for it out of pocket, but that would probably be a waste of money in my opinion.
In those with healthy hearts, occasional PVCs and PACs are harmless and usually resolve on their own without treatment. Some can be managed through lifestyle changes, such as limiting caffeine, tobacco, alcohol, and stress. Those who experience arrhythmias on a regular basis may benefit from medications such as beta blockers and/or calcium blockers, and in most severe cases, interventions such as catheter ablation.
Criteria for diagnosing ARVD/C was initially proposed in 1994, then updated in 2010: This chart compares the old and new: https://www.hopkinsmedicine.org/heart_vascular_institute/clinical_services/centers_excellence/arvd/_pdf/arvd_checklist_comparison.pdf. You can use the column on the right, and check to see how many major and minor criteria you meet.
Definite = 2 major OR 1 major + 2 minor
Borderline = 1 major + 1 minor OR 3 minor
Possible = 1 major OR 2 minor
Based on your post where you stated your family history and tests thus far were unremarkable, aside from the PVCs and PACs, I suspect you barely meet any major or minor criteria. That said, you still might benefit from the ambulatory ECG monitoring and/or cardiac MRI, which can rule out other pathologies even if you do not have ARVD/C.
Arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C) is a rare familial disorder that may cause ventricular tachycardia and sudden cardiac death in young, apparently healthy individuals. The clinical hallmark of the disease is ventricular arrhythmias, arising predominantly from the right ventricle. It is often difficult to diagnose ARVD/C, as there is no single test that can either establish or exclude ARVD/C. The diagnosis of ARVD/C is based on meeting a set of specific criteria, including global/regional dysfunction/structural abnormalities, tissue characteristics, repolarization and depolarization/conduction abnormalities, arrhythmias, and family history/genetics.
If you suspect ARVD/C, you should share your concerns with your cardiologist. A reasonable initial approach might include a thorough family history as well as a 12-lead ECG, transthoracic echocardiography, ambulatory ECG monitoring, and cardiac MRI. You can discuss with your doctor if you may benefit from ambulatory ECG monitoring and/or cardiac MRI.