One the I-130 is approved, it is up to you to move things forward. The I-130 is approved for an indifinate period of time, though the US Embassay will usually put it on hold after a year and you have to jump through hoops to get them to dust it off.
You need your case number (LND) to book the medical and usually the Knigtsbridge doctor's can see you within a week or so. You medical has to be within 6 months of when you plan to enter the US though. So if you had it now, you would need to enter before the 6 September 2016 (it isn't from when the entry visa is issued)
You need to have a UK Police Certificate in order to take the medical, and it has to have been issued within 6 months of having the medical. If you haven't ordered it yet, do so now. The normal timeline is 2 weeks for the Certificate.
Once you have the date for the medical and submitted your DS-260 (which you can't start until you have your approval) you then notify US Embassay you are ready to proceed.
My DH husband had his medical on the 29 Fb and we can see on the USCIS website that we are now awaiting the appointment date. From what we can tell, it is usually about 6 weeks from the medical.
