As said zero as the testing and inspecting is all part of the job, it is not a separate cost added should the customer require it.
However with LABC I realise they often insist some one other than the person doing the work tests it. This happened we me, in the end I did test it, but that was not what the inspector wanted. In the main it would be the minimum charge. For any job you must travel to and from the job, and in the main electricians work on a swings and roundabouts system charging what on average it takes to visit. In rural areas electricians can work quite a distance from base so around where I live normally 2 hours charged, but this will vary.
Since likely two visits required that means 4 hours, this is why so often we say simply not worth doing DIY on a job needing registering. Half a day is also the time taken to do the job, so it costs £100 more plus vat to DIY than to get an electrician to do whole job. OK if full house rewire then DIY but single circuit if following the law then a non starter.
OK it likely takes just 15 minutes to do the work, however it's the travel time that bumps up the price.
And after paying all that money your still left without an installation certificate. Only the person doing the work can issue an installation certificate, so some one who just tests issues an EICR (Electrical installation condition report) and the completion certificate is issued on the strength of that report.