I doubt they do the chroming themselves. It's a very 'hazardous' process (from H&S chemicals perspective) so generally only specialists tend do it these days. There's probably another margin involved in that case.
I think it even says something about them outsourcing the chrome on the site. No objections to that of course.
It may be better to go via them than to go directly to a chrome plater since they presumably have the knowledge to make sure the chrome's done properly (e.g. that bottom bracket threads don't get plated )
In any case, I've just had a quote back from a place here in Belgium that says it can do the same work for about 300 quid but that they can also source original decals, which also seems very reasonable.
I wouldn't think any framebuilder do their own plating, except for the very large volume producers, eg Peugeot etc, but they're not usually callled a framebuilder.
A lot of framebuilder don't or didn't even do their own painting.
It was my job as a youngster to deliver / pick up frames and bits that had been painted and / or chromed from our refinishers. Wenlock Platers in Hull. Amazing what you can carry slung over your shoulder on a bike...........no car in those days. The Plating part looked pretty nasty and can't believe you could have that sort of stuff going on in the back of a framebuilders shop !