there are 3 different combined scans used for screening for DS.
There is a first trimester screen which involves a bt at 10-11 weeks and the scan at 12 weeks. There are also triple and quad screens that you can get where the blood tests are taken later. Which one you have depends on you OB/nidwife, and to a dregree what you ask for or consent to.
12 weeks scans are not required but are routinely offered by many carers.

Where you can go to hospital depends on the hospital - some have intake areas and some are first in first serves, so ring up the one you want to go to and see whther you need a referral to book in or whether you can just do it yourself.