Legally new bikes have to be sold this way, although you're allowed to change it yourself.
Erm, that's just not true. Hop on a Trek at your local bike shop and look at what way round they are. There's no law that says "left/back"
You may be right, I think it's more a custom rather than legal requirement. I have the full standards for Mountain Bikes I assume it to be the same for Road Bikes.
Mountain-bicycles - Safety requirements and test methods
A bicycle shall be equipped with at least two independent braking-systems. At least one shall operate on the front
wheel and one on the rear wheel. The braking-systems shall operate without binding and shall be capable of
meeting the braking-performance requirements of 4.6.7.
Brake-blocks containing asbestos shall not be permitted.
4.6.2 Hand-operated brakes
126.96.36.199 Brake-lever position
The handbrake levers for front and rear brakes shall be positioned according to the legislation or custom and
practice of the country in which the bicycle is to be sold, and the bicycle manufacturer shall state in the
manufacturer’s instructions which lever operates the front brake and which lever operates the rear brake (see also
Clause 5 a)).
I don't know whether there is any separate legislation regarding lever position, but certainly as far as the EN (replaced old BSI) standards show there is no requirement.