I am reasonably sure that something in recent UK planning law has changed and it is now illegal not to use a qualified electrician even to install your own mains wiring for garden lighting

It's the so called "part P". As far as I understand it, it doesn't prevent a "DIYer" from carrying out the work you mention but to go about things that way, Building control needs to be notified before the work is started and the work has to be inspected after the work is completed. I'd imagine by the time one is done and paid the fees, one might as well have just paid a qualified electrician to do the job.