I'd be very wary about relying on normal home insurance to cover workshops, built-in or otherwise. My double garage is attached to the house and is linked into the alarm, but it still isn't covered. Way back my home insurance only covered £2k in garages to cover the sort of thing most people have like lawnmowers and DIY power tools. When I asked about cover for other items like a lathe and mill all I got was a silence, and then a no we don't cover. In other words they had no idea what they were and it's easiest to say no.
I have a friend on another forum who confidently told me that his detached, but brick built, garage and workshop was covered by his holdhold insurance up to £30k. A couple of years later he sheepishly told me his wife had made some comment when renewing, and the insurance company had said nothing was covered, and never had been.
My workshop machines, tooling, part built engines and materials are covered by Walter Midgeley. They're by no means cheap, but they seem to be only player in town. A couple of years back when I thought about changing I couldn't find anyone else other than a few one man and his dog brokers who all led back to WM..
Andrew