Hello, welcome to FlyerTalk.

Here are the T5 A380 routes, and the Avios needed for F redemption (on peak dates, knock off 10-20k for off-peak).
Hong Kong - 120k each way
Johannesburg - 100k each way
Los Angeles - 100k each way
San Francisco - 100k each way
Singapore - 140k each way
Washington - 80k each way

(there is also Miami and soon Vancouver, but they go from T3)

Availability is hard to come by on several of those routes. For example, right now it appears LHR-SFO has only one date in 2016 with two F seats available for redemption (and that is in November). So you may need to be quite flexible.

If you are looking for a long flight, a cocktails-on-the-beach connection and a nice lounge on the way back, Singapore is probably one of the better options. But note the round trip is slightly more than your Avios balance, even without the ARN-LHR and SIN-XXX flights. I would also have a look at Kuala Lumpur - 787-9 rather than A380, but some say it's a better experience, and you will be able to connect on to MH flights to surrounding areas.

I am afraid I do not know the answer to your two questions, but I am sure someone who does will be along shortly.