Where else do you do the joint so it will not be disturbed?
Using the appropriate Junction Box to create a joint in a cable underneath a floor is more in breach of 7671 than bonding bathroom pipes.
In an ideal world all joints would be readily accessible for future inspection, however we do not live in the real world.
I will agree that all steps should be taken to avoid jointing any cable, alternatives are always preferable, but there are times when this is simply not a practical option.
Regulation 526-03-02(i) states:
A suitable accessory complying with appropriate British Standards.
A Junction box will comply with BSi requirements or it would not be for sale.
Regulation 526-04-01 states:
Except for the following, every connection and joint shall be accessible for inspection, testing and maintenance.
It does not say you cannot put a joint beneath floorboards. I accept that accessibility may be an issue, but it is not as if you are sealing the joint for eternity.
The Regulations are a GUIDE that must be adhered to, however they MUST not be read literally in such a way as to create unecessary work or situations were people may be tempted to take short cuts around a problem due to adherance to a strict and unbending regulation that has been too literally interpreted.
If the Regulations clarified what they mean by "accessible" then perhaps this conversation would be a moot point, but as usual they are a clear as mud.
The Regulations must be used in a practical manner in the real world, complying with ever part to as high a degree as is practical in that situation, if they are too inflexible for this, then they will be ignored completely, then we have a serious problem.