Put in simple terms, you should not fly in a London park, as they are pretty much ALL considered to be congested areas - so the 150M rule applies.
The rule states you must not fly over or within 150M of any congested area. Unfortunately that means that virtually all of London is out of bounds for quadcopters fitted with cameras. There are also issues with most of central London being controlled airspace, again making flying illegal, as well as bans on flying in all the central parks under their own bylaws.
Best bet is to find a local flying club, who will have legal flying sites and can help with some training as well. These quadcopters are really nice and easy to fly, until things start to go wrong, then you really need to know the basics of flying to recover without a crash. A few hours with an experienced pilot will help a lot.
I also don't buy the interference issue only giving you 200M range. Check you don't have any height or distance limits set in the software. I fly on an old air base, with high power radio links from the active USAF communications site next door and can get 600m - 1000M without a problem. I would need to get very close to the transmitters to start having a problem. 2.4GHz Wifi would also need to be quite close to give you a problem as well, as the base stations are less than 50mW each.You need a single big transmitter to swamp the receiver with RF noise to cause a blackout, so a big TV transmitter broadcasting on thousands of watts or similar. Frequency is less important that power when it come to interference.