Generally spoken: No plaintiff, no judge. Which means, you may violate a lot of codes and regulations, but nobody will care, as long as you don't stir up too much dirt.
CE is a set of rules you have to follow, when you like to sell something in the EU (to which you still belong). As long as you don't sell things, you don't have to state compliance with those regulations. OFCOM will only kick your ass, if someone complains about your EMI.
But with electricity there are some nifty details, as you should know.
- Electricity can kill people
- Electric devices (exempt incandescent bulbs and toasters) emit electromagnetic radiation
- Malfunction in electric devices can start fires and damage cabling and other devices
If you follow all rules and codes, you have already done much to fend off any detrimental effects of your DIY project. This does not save you from nasty legal consequences if your devices fail despite all countermeasures you have taken.
If you aren't an engineer who has an insurance for the risks connected to your job, you may be held liable for damage, even if codes were followed thoroughly, because codes and rules are only covering a part of all possible mistakes. Only people with sufficient insight (i.e. engineers, electricians etc.) are considered to be aware of all lurking risks. If you aren't one of these, you ought to know, that electricity is dangerous and you shouldn't fiddle around with it.
Implications may be:
- Fire insurance refusing to pay
- liability for personal injury (not as expensive as in the US but painful nevertheless)
- custodial sentence possible in case of personal injury
- monetary fines due to violation of OFCOM rules. Usually only if someone complains about the EMI you were causing.
But if you decide to do your own design and decide not to comply with all rules and regulations you have to take some additional risks. This applies to people with sufficient knowledge even more. So if you are an engineer (who should have known better) and build a device which harms people you will encounter no pity in front of a judge.
Additional implications, if you should have known better:
- fire insurance will refuse to pay in case of fire caused by equipment, for sure
- private liability insurance may refuse to pay if you knowingly violated codes
- accusation will be concerning gross neglegience instead of slight fault.
- even the assumption of wilful intent is possible.
- loss of job (as your professional seriousness is in doubt)
- occupational ban
A good start for not risking jail and a life in poverty and remorse is to use off the shelf walwarts as power supply and only deal with SELV circuitry in your own design. Then it will be rather unlikely to kill or harm people with your circuitry. The EMI-risk is per se rather low as long as you do not try to build your own oscillators and wireless transmission devices.