NO YOU HAVE NOT UNDERSTOOD!!
The Owner of you on behalf of the Owner must register with building control BEFORE you start work - not during or after. The amount you pay to Buidling Control is dependant on the real value of the work being done (no false prices!!).
You have to give building control an opportunity to see the installation a) before you start, b) after first fix, and c) on completion with all the test results - BUT BEFORE BEING MADE LIVE for user.
You will also need to show compliance with the various sections of the Buidling Regulations:
B - Fire Safety
C - ... Resistance to Moisture
E - Resistance to the Passage of Sound
F - Ventilation
L - Conservation of Fuel and Power
M - Access to and Use of Buildings
P - Electrical Safety (of course)
BTW:
It is a criminal offence to carry out work that does not comply with building regulations, with a maximum fine of £5,000 - the Owner would be liable for this offence!