Hi Duncan
BSF have been introduced into Southern Europe already and certainly seem common in this part of Portugal. They really need temps of around 28c or more to breed well so I don-t think they are viable in the Uk. I know somebody in Wales who had them do well wild in his garden for a while, but they didn't last long.
You might have luck with a
greenhouse, but these flies traditionally need
alot of space to breed well. I tried a 2x2x2 metre mesh cage here, and breeding was very poor. But I have reasons to believe captive BSF available in the UK are semi domesticated and are less fussy about space. I tried wild strains.
If you want some 'search engine' words such as 'calci grubs' or 'calci worms'. They are now commonly availabble as reptile food and can be ordered in the post.