Hey Nessie,
also from UK, and yes things like rosy's are about £2 each,
Mice pinkies are about 55p from pet shops or about 20p online... so you Americans can imagine how expensive it is to feed our snakes...

I buy mice and pinkies in bulk online from icemice...(pinkies down to 19p) and have a draw in the freezer just for frozen mice!!
and feed both mice and chopped up trout.

Trout has NO thiamanese apparently, I buy it from Asda, chop into bite size bits and feed with mice, sometimes alternately, sometimes mixed..

All my snakes eat both.

I have not used a suppliment ever (well since the 80's when I was feeding on whitebait) But all snakes lively and healthy..

Oh and I give them the odd garden worm (what the americans on here call night crawlers, threw me at first, I thought it was some foriegn insect that they ate!) just make sure its the common garden thing and not the redder ones that you find under sacks or in composts as they are toxic to your snakes..