Agreed. You don't have to have it tested by a specilist. However the law does say that you must be confident that it's compliant, and you must place the sticker on the system, as well as keep documentation. Also, it is an offence to place a sticker on a system which you know is not compliant. The thing I'd be worried about is how do I justify that it's compliant with the EMC directives? I don't think
"I just have a feeling" will do...
Granted... If you were just producing computers for home use, the chances of anything bad occurring is unlikely (and I mean that in a legal sense, not just electrical), but you say below that you might make a rack mounted firewall, and well, now you're no doubt offering your products to other companies, and your equipment will be running alongside their important IT operations. Down time is not an option.
The chances I feel are still very slim of anything happening, in all my time building computers/servers I can't remember having had an issue, but its that chance of causing a company to stop work, because your product is not perfect. The cost of this could be very high indeed for a small business.
I would perhaps find out what it would take to properly self-certify these systems from a legal angle. I personally am qualified to PAT test, which was a service we would sometimes call in on behalf of our clients, but it was easy enough to learn with just 3 days at a college. EMC testing sounds a bit harder and more expensive to implement, but I'd try find out if I was you, in the long run you will be better off - and it might grow into a service you can offer to other small IT companies.
I see where you are coming from, however I'm not really "in" the hardware business. I make most of my money selling support and maintenace services. I would only build a few custom unit when ones that are on offer to buy won't do. An example of this is a custom rack-mount firewall. Since I'll only be making maybe <10 systems a year, it doesn't really make sense for me to pay £600/year to a Producer compliance scheme...
Hard one really... You do seem to be hit by the regulations much more than I was.
I can only really refer to you someone who I hope can help you. I'd give Valpak a call -
http://www.valpak.co.uk