Rush hour is a very bad idea. When I booked my test during rush hour my instructor immediately told me cancel it if i could. The problem with rush hour is that everywhere is 10x busier. That may not be a problem if you live in the countryside but on the outskirts of London things get very slow and tedious. Examiners want you to 'make progress' and so when you're at a junction and no-one will let you out or give way, the only other option is to force your way out or cut someone up, which is a fail. However, if you sit there waiting for a safe gap you can get failed for undue hesitation.
It is all at the discretion of the examiners. They may accept that it's more difficult in rush hour and let you pass, but there are no obligations. It is crazy that while the conditions of the road always change, the marking system doesn't. Undoubtedly there are good drivers that are failed in rush hour conditions, and poorer drivers who benefit from easier driving conditions later in the day that pass. Then there are those that take their test in rural areas and whom are scared of driving in cities or on motorways, that get the same pass as someone who has taken their test on the outskirts of London and who are happy to drive anywhere. Am I the only one who thinks there's something unfair about that?
Ideally get a test during the day when its quiet. You've got a much greater chance of passing (statistics prove it), it's easier for you and cheaper if it means that you pass quicker. By all means have lessons during the rush hour so that you get used to the worst case scenario with a greater traffic volume , less considerate drivers desperate to get home, etc. That is what I did by having lessons straight after school. It will make you a better driver, but for the test get one when its quiet!