You will get the TPs for each sector you have flown, including PRG-LHR. When you go through the gate on boarding the aircraft - that is when the TPs are sent into the computer. This can be reversed (e.g. if the flight is cancelled when you are all sat on board and ready to go) but the defaut is that the TPs and Avios will stick at that point. It's just the last leg you won't get if you don't fly it. It's not quarantine by the way, which is something different, it's self isolation.

For the fourth leg, you can't have an FTV. Your only option is to move it to another date, paying the fare difference if any. Careful scrutiny will find you a date when there will be little or no extra to pay. You don't pay the change fee which is usually the expensive aspect of doing this. You can apply for a voluntary refund but it may not get much money back.

At the lounge or at the customer service desks, by World Duty Free / Starbiucks, you can have a discussion with an agent about dates, indeed you can get the agent to locate your fare bucket and come up with another travel date when that fare bucket is currently on sale and so miminise the cost to you. If you prefer the refund you can get an agent to cancel your booking to avoid the no-show and then apply for a refund at your leisure.