Michael MacMillan said:

• Is this an acceptable installation method under BS7671?
• What specific clauses in BS7671 would these pieces have to conform to (511.1?)
• is there anything in the Regulations that would allow this installation to be rejected?

It’s amazing sometimes how a contractor’s cock-up can become someone else’s problem. I am usually sympathetic to contractors, been there and done it, so to speak, but I would stand my ground on this one. Grand if there was a specific technical reason to reject it but for me it is a contractor mistake that fails to meet the design intent. 
When the contractor leaves site, the client is left with something that was never intended. The contractor will have forgotten his mistake in short shift, but the mistake will be there for the whole life of the installation. Who knows, there may even be  a query raised at the first periodic inspection, which the client will have to deal with. Why would you accept the bother?
What technical argument could you put forward, for example, to a spark who leaves the socket outlets and other accessories at different heights above the worktop in a new kitchen? 

Michael MacMillan said:

• Is this an acceptable installation method under BS7671?
• What specific clauses in BS7671 would these pieces have to conform to (511.1?)
• is there anything in the Regulations that would allow this installation to be rejected?

It’s amazing sometimes how a contractor’s cock-up can become someone else’s problem. I am usually sympathetic to contractors, been there and done it, so to speak, but I would stand my ground on this one. Grand if there was a specific technical reason to reject it but for me it is a contractor mistake that fails to meet the design intent. 
When the contractor leaves site, the client is left with something that was never intended. The contractor will have forgotten his mistake in short shift, but the mistake will be there for the whole life of the installation. Who knows, there may even be  a query raised at the first periodic inspection, which the client will have to deal with. Why would you accept the bother?
What technical argument could you put forward, for example, to a spark who leaves the socket outlets and other accessories at different heights above the worktop in a new kitchen? 

lyledunn said:

What technical argument could you put forward, for example, to a spark who leaves the socket outlets and other accessories at different heights above the worktop in a new kitchen?

Technically, it does not matter.

Don't get me started on tradesmen. I have been spending much more time than I wanted recently replacing some badly corroded steelwork which supports my conservatory. The steel-worker wanted to get out a road breaker to hack out the old stump and plant a new one in the ground. Granted, that would outlive me, but that is not the point.

In any event careful digging revealed that the stump was bolted to a pair of 4" angle irons 12" down on the foundations. I have not found the other end. So you can imagine the destruction that would have been caused.

If you ordered 2.4 m of steel T-section and only 2.35 m arrived, would you accept it? Of course not!

ETA: I suppose that it is all a matter of contract.