A general point for MOT testing, unless compulsory, is "where fitted".
e.g. If you have a sun visor fitted it will/should be tested. If it flops down easily then you have a problem. However were you to fail you could remove the visor and reapply for testing. You will then be OK.
Now with regards to seat belts it may be a little more complicated.
After a certain date all cars with active passenger seats had to have seat belts and after another date they had to be self retracting (all trigger/based on vehicle production/registration date). So a 5 seater 2010 car I suspect WILL fail the MOT if seats are present but no seat belts are present. Remove the seats and I suspect you will have not problem.
An other pont to bear in mind is that an MOT is valid ONLY at the time of testing. So if you remove your floppy sun visor for the test and then refit it after the test and have an accident or otherwise get inspected then your vehicle (subject to obvious crash damage etc.) is assessed on roadworthy status at the time of the investigation. A classic example of this would be defect or bald tyres.
Another odd twist I seem to recall (from my motor sport days) is if you have a road legal single point/buckle release harness fitted (as opposed to a multi point buckle non legal road use harness) and still retain the normal seat belt then both will be testing and an issue with either will fail the MOT.
My advise to anybody, beside doing the research and legwork yourself is to visit you Local County Council MOT Testing Station. All councils have them and provide both Taxi/commercial and public testing by law.
These local government MOT testers will give you the best, most open and honest facts about MOT testing issues/criteria. They only do testing, they only apply rules and regs with fair discretion and they don't look for income revenue. If you fail they will never recommend a specific garage to get your car fixed.
My local council testing station will answer any of your MOT related question free of change and impartially. They will also will also, free of charge, check things that for us self maintenance people we can not reasonably be expected to be able to test ourselves. e.g. vehicle emissions, brake balance or headlight aim. These tests require specialist equipment and they will run the tests but you may have to book and they certainly don't accomodate fools wasting their time with clearly obvious issues that even the local cat could address.