Chicken breast defrosted and in fridge 2 days, ok?

If it had time left on use by before it was frozen and then defrosted, kept in fridge I'd have no problem using it as normal.

Why is that exactly - I always freeze meat/poultry with days to spare on the use by date and usually will eat on same day I defrost but on many occasions its the next day - its never made me or my family sick.

Perhaps share the science/food hygiene reason - OP said it defrosted in the fridge and stayed there since, so it has never been at 'room temperatures'

I didn't think any food safety advice of any kind was allowed?

handy having a dog in these situations - she never says no

No it wasn't, you were confusing 'best before date' and 'expiry date'. They're not the same and I'd recommend you familiarise yourself with the difference.