I am a bit unclear on a few points?
(1) Do you mean you are considering offering an estimate / quote for doing the work...?
(2) Do you mean that work has already been agreed and you somehow need to figure out how to do it..?
(3) Are you self employed... if Yes and (2) is yes, then how have you agreed a cost?
(4) Are you self employed... if Yes and (1) is yes, then walk away.. probably too much hassle for minimal return!
(5) Are you an employee/sub-contractor who has to sort out someone else's crap.? If yes then pass the buck back to them for how you are supposed to do it??
(6) Although the light brown beans may be superficial.. there will be some real beams/joists between the plasterboard and the roof that will need negotiating your cables around!!!
(7) To my limited knowledge, quick-wire only helps with the connection bit.... It will offer NO help with trying to get the interconnecting cables through "Solid-Insulation" and past Joists, between each of the lighting points..
(8) How are you going to ensure your 6x downlight positions are all nice and symmetrical and in-line, and NOT clashing with the real ceiling joists?
(9) How do you know what clearance depth you have to accommodate whatever downlights you choose?
(10) IF it is feasible to do for a decent cost.. I would probably use the JCC fireguard cans with a suitable bezel and appropriate GU10 LED lamp.. e.g. Cool/Warm, Lumens, Beam angle, Dimmable etc.. etc..
https://www.jcc.co.uk/en_GB/products/jc010010/nb
(11) If you do go ahead with this job.. and you can get some decent money for doing it.. then I would think multiple pilot holes and larger access holes for feeding cables will be needed in addition to the actual 6x downlight holes!
(12) Pain-in-the-bum jobs can be done providing you are happy with the hassle.. and the customer is willing to pay a premium for your hassle! So you need to evaluate all the pros & cons, and don't sell yourself cheap!!! As I often say to customers.. Anything is possible.. providing someone is willing to pay you for the actual hours needed to undertake the work!
(13) Surface mounted downlights sounds like a bad-idea and contradiction of concept for ceiling mounted downlights!!?
hope that helps?