AJJewsbury: 
 

I worked with an electrician who always used them on lighting circuits with 6a breakers, is this common practice on domestic properties?

If common practice is anything like what I do, then it'd be more like avoiding junction boxes like the plague.

I much prefer to keep terminals accessible - looping in at lights or switches - is much preferred.

Junction boxes might sometimes be needed for additionals/alterations to existing circuits - though to be honest I'd probably go for a plastic box and some wago connectors these days. I can't remember when I last used a traditional round JB - certainly more than a decade ago.

    - Andy.

Sometimes the small circular junction boxes were good as they fitted through a hole in the ceiling allowing a new downlighter to be wired in. Also they were flame retardant and not just made of flimsy soft plastic.

Z.

AJJewsbury: 
 

I worked with an electrician who always used them on lighting circuits with 6a breakers, is this common practice on domestic properties?

If common practice is anything like what I do, then it'd be more like avoiding junction boxes like the plague.

I much prefer to keep terminals accessible - looping in at lights or switches - is much preferred.

Junction boxes might sometimes be needed for additionals/alterations to existing circuits - though to be honest I'd probably go for a plastic box and some wago connectors these days. I can't remember when I last used a traditional round JB - certainly more than a decade ago.

    - Andy.

Sometimes the small circular junction boxes were good as they fitted through a hole in the ceiling allowing a new downlighter to be wired in. Also they were flame retardant and not just made of flimsy soft plastic.

Z.