The alternative to the treadmill if you cannot complete it is the gamma drug one where you are injected with a radio active die and a drug which increases your heart rate (puts it under a drug induced stress/workload)
You then lie is a machine similiar to a CT scan for 20 minutes whilst it takes a 'video' of your heart beating.
You then have to gp back in a few days time and repeat the scenario without your heart being stressed and they then compare the two results.
I passed mine but you have to do it again to renew in three years and I could not be bothered again, it costs the DVLA a lot of money.
The only reason I had to do it was because I had a couple of stents in my groin, nothing to do with my heart, it was blood circulation to legs !!
Peter