After 1 dose of mmr, it is over 90% effective, the 2nd dose helps provide immunity to another few %. They are planning to bring the 2nd MMR forward to 18 months nationally. If a child has the 1st dose between 6 and 12 months, they should have another dose at 12 months and then another dose, so 3 doses total (so if you have a missing dose on your record, you can have a 3rd).
You can still get breakthrough measles, if you have had 2 doses, but it is considered to be very low risk of being transmitted to others with healthy immune systems.
Measles is diagnosed in high amounts by gp's currently, all cases should recieve an oral fluid kit in the post which should be completed and returned to confirm or not.
You only need to isolate for 5 days from onset of rash for measles even if rash remains - if exposed and not immune, it can take 21 days for infection to start.
Rubella is no longer tested for antenatal now as there is very very low levels of rubella in the uk (norimported from travel to other countries with circulating rubella), so with no natural exposure immunity levels difficult to check (doesn't mean no immunity if you were exposed)