Firstly, the Client is not responsible for acceptance under the contract which is the role of the Project Manager.
It is not clear from what you have said whether the British Standard requirement formed part of the Works Information / Scope and whether the Contractor had any design responsibility.
If the BS requirement was part of the WI/Scope then, assuming the Contractor had responsibility for design, the submission would have been a proposal by the Contractor to change the WI/Scope. Although this would not constitute a compensation event, the PM would have been obliged to instruct a change to the WI/Scope or the matter would automatically lead to a Defect. If the weld failed then you would consider the Contractor's design liability under the contract for any redress.
If the Contractor did not have any design responsibility and proposed a change which was accepted, then the PM should have instructed a change to the WI/Scope accordingly, which would be a CE. In this instance the WI/Scope would remain the responsibility of the Client, including the change. Note that the Prices would be reduced under Options A and B but not under C and D.
Essentially it depends on whether the Contractor had any design responsibility under the contract for the welds / connections. If there was no design responsibility then the Contractor is to Provide the Works in accordance with the WI/Scope. Any proposals may have led to reduced Prices as a CE, but the scope requirements would still be the responsibility of the Employer / Client..
You would likely pursue, however, any issues which may mean the work constitutes a Defect,, although if the WI/Scope (as amended) required something that didn't work, then the Contractor is obliged to provide this !!