And just to confirm that... there's always been a very straightforward definition when it comes to the 12 months for the driving licence:
The year begins from the date you began your residency in the UK... you moved to the UK on your settlement visa and became resident in July, so the 12 months started in July.
It doesn't matter that you went back to the US for a visit in October - your primary residence was still the UK at that point.
The only way the 12 months would have started in October would have been if you entered on a visitor visa in July, then went back to the US in October, applied for a spousal visa and then returned to the UK.
From the direct.gov.uk website (
http://www.direct.gov.uk/en/Motoring/DriverLicensing/DrivingInGbOnAForeignLicence/DG_4022561):
Residents
If you are the holder of an ordinary driving licence or a valid international driving permit, you can drive any category of small vehicle shown on your licence. You will be able to drive for up to 12 months from the time you became a resident.
To ensure continuous driving entitlement a provisional GB licence must have been obtained and a driving test(s) passed before the 12-month period elapses.
If you obtain a provisional licence during this period, you are not subject to provisional licence conditions eg displaying 'L' plates or being supervised by a qualified driver or being precluded from motorways.
If you don’t pass a test within the 12-month concessionary period you cannot drive as a full licence holder, and provisional licence conditions will apply.
If you wish to continue driving you must apply for a British provisional licence with a view to passing a driving test. Provisional licence conditions will then apply.
If you don’t apply for a provisional licence within the first 12 months you must stop driving.
New residents who hold a vocational licence (minibus, bus or lorry entitlement) must not drive large vehicles until they have passed the relevant GB driving test.
Driving test candidates are required to pass a motor car (category B) test first before applying for provisional entitlement for larger vehicles.
Edited to add: In terms of being considered ordinarily resident, the 185 days part essentially just means that you have permission to live in the UK for more than 185 days - either by being a UK/EU passport holder, or by holding a visa that is valid for more than 185 days (i.e. someone on a visa valid 6 months or less would not be considered a UK resident).