If you aren't already receiving an annual statement regarding your US social security, ask for one shortly after you return to the UK (and be sure to give them your change of address at that time).
About 2 or 3 months before your 65th birthday, go in to the US Consulate in London to discuss your social security/Medicare situation and get signed up for everything. (If you really want to start drawing benefits at age 62, contact them a couple months before you hit that birthday.) This is the current Federal Benefits and Social Security page: Federal Benefits / Social Security | Embassy of the United States London, UK
(It may change by 2019, but handy to have for the moment.)