A cheap blood pressure monitor is all that's needed to spot signs of arrhythmia, as they all seem to include a function for spotting it these days. I use one of these
https://www.ebay.co.uk/p/501438974 , which does tell me from time to time that my heart rhythm is irregular. However it's only telling me that I ought to get checked out by a doctor, not that I need to call an ambulance.
A more accurate method of monitoring heart activity is one of these (together with a compatible smartphone or iPad)
https://store.alivecor.co.uk/products/kardiamobile
which appears to be endorsed by research from the British Heart Foundation:
https://www.bhf.org.uk/what-we-do/news-from-the-bhf/news-archive/2019/march/smartphone-ecg-could-be-used-in-ae-to-detect-serious-heart-conditions
However it's not clear to me how useful the data is to a layman and, in particular, how it can be used to differentiate (again, by a layman) between "I ought to get a check-up soon" and "I need an ambulance right now!"
Apple Watches are far from cheap but their Serie 4 models do come with an ECG feature built into them
https://www.thesun.co.uk/tech/8732260/apple-watch-ecg-uk-afib-how-to-use/
Once again though, it's not clear to me how one differentiates between a level of irregularity that simply needs checking out by a doctor and one that requires immediate attendance by paramedics.
A call to the BHF's Heart Helpline might provide you with advice as to whether there are any monitoring devices which can give you exactly what you want from them. Calls are free, from both landlines and mobiles, with the lines open from 0900 to 1700, Monday to Friday. Their number is 0300 330 3322.