It is proposed that the CAA gets £534 for each test that is carried out by a CAA Authorised examiner. Then the examiner charges an 'appropriate' fee determined by each examiner, PLUS they will be allowed to charge expenses.
AND the CAA will allocate the examiner who is to carry out your test and you will not be allowed to change him/her.
Rumour has it, if you do not do the test due weather, sickness, technical issues, etc you will still have to have the same examiner - when he/she is next available.
In summary, if you get an examiner who charges, say, £400 plus high expenses it will cost you a lot more than £800 and you will be able to do nothing about it.
That's the way the UK CAA have run JAA CPL testing in the United States for around the past year or so, except it was just travel expenses.
You can request a change if the examiner is not available within a 10 day window of your request for a test which can only be made once you have your 170 in hand, as they require a copy to allocate the examiner.