[Closed] HRM wanted - which to choose?

Have been using a Geonaute watch from Decathlon, now about 5 years old and the strap is on it's last legs and unreplaceable. Using it for training on the bike a few times a week and key statistic is average HR.

Most important features I want:

visible average HR while training (not just after)
actual HR while training
actual time spent training while training
self-replacement batteries for watch and chest strap

Budget is around £80.

Have looked at quite a few particularly Polar FT4 but not many seem to offer the features I want. Many offer the data after the training session ends, but that's not for me. I need the data as it happens so I can up or down my effort.

Anyone got any pointers?