Schiphol reconfigured their security pcedures a few years ago, such that passengers arriving from "trusted" non-Schengen countries (which includes the UK) don't have to pass through transfer security but deplane directly into the non-Schengen concourse. So you will not need to wait in line for either security or passport checks.
Thus, if KLM is willing to sell you a 55-minute connection, it sounds perfectly doable, assuming your inbound flight is not late. Don't expect to make it to the gate before the "boarding time" indicated on your boarding card, but you'll get there before the doors close. (So don't expect to be able to meet your companion at the gate, and expect that you may have to fit your cabin luggage into the space left in the overhead lockers after everyone else have had their go).
It's possible that you make it but your checked luggage won't. Having a change of underwear for the first night in your hand baggage would be a smart move.
Also, as Michael Lugo mentions, note that KLM has flights from Heathrow to Amsterdam practically every other hour on a typical afternoon (as well as additional departures from London City) -- so if you want to minimize your risk of trouble, just book an earlier flight.