I'd love to see pictures :)
Sounds like a nice little history...have you looked further into your background of the garage, might be some interesting stories behind it, maybe some famous people used it, you never know!
Not too sure about certificates, but if you go to English Heritage they may be able to help and grade list your building.
Also if you go to local newspaper or even national papers, they can run a nice little history spread about your garage, put photos up and they may be able to look into further backgrounds of it, plus it increases business. Sometimes people who are writing history books of the area etc. might want to put your business in and research it.
Hope this helps
I have some pictures now.. (sorry about the quality though)
okay,so the old clips we would put in the sales cars to show we supplied them

this is how we used to look.. the buildings on the left is now the front, with the shopfront and petrol/diesel pumps.

the ford sign is us. this was about 70 years ago when we were a ford franchise dealer. after that we became a mg/Morris/austin dealer.. (ONLY FOUND OUT TODAY WE USED TO SELL BRAND NEW MINIS)


this is a copy of local business ads.. I will look for a date sometime..

(zoomed into ours)

finally south molton square..

I'll post a pic of us now tomorrow