My own personal opinion : for myself, I look at the total cost of getting a bottle filled (which includes direct travel costs and time). My local shop (8 miles away) charges £5 for filling small bottles and £8 for filling big ones, along with £45 plus fill for a full retest (an additional £8 in my case).
I'm happy to pay that sort of charge as I quite well believe them when they say that Bottle Filling is a Loss Making Situation for them due to the high cost of running a Compressor and the lack of incidental sales to Airgun Users who pop in for a Bottle fill only and nothing else.
My Club has their own Compressor and based on running costs (and depreciation) over the last two years I'd guess that filling a 12 Ltre Bottle costs maybe £15 or more, they are not a cheap item to run and few people appreciate the true cost of running them.
If your dive shop is five minutes drive away you'd struggle to make any real savings by driving a lot further and saving £5 in the process.
JMHO, Vic Thompson.