You can ring up now and get the PRG flight cancelled and off the itinerary, you would get a small refund for that component (£40 perhaps?).

Or you can leave it, and ask it to be rebooked for later on, subject to the ticket validity rules (essentially at least 1 year from ticket purchase). This usually is the best value for money.

You can also move it to another location within 300 miles or so, but keeping to approximately the original date of travel.

Or you can cancel the ticket at LHR / MIA. There is a slight risk here. If you can OLCI then it's fine to do this, but if you can't then you need to call up and deal with the PRG sector.