Hi Zulberg,
Posts like this are hard to answer as everyone location, house, tiles etc.. are different. However being in London you will be guaranteed to see a lot of traffic from overflights from Europe, arrivals/deps from Heathrow, Stansted, Luton and London City even Gatwick if your have a good location.
You should be able to get better reception in your loft then at windows level on the 1st floor as simply being higher means less obstacles around.
With a roof antenna there a lots of alternatives, have a look at our dealers who should be able to pick out the exact set for you and the connectors and anything else you may require to mount it.
A good external antenna and preamp (and good location) would mean you would receive a good 200nm range all around as London and surrounding areas are pretty much flat except with the Downs in the south. Have a look at the screenshot forum for shots from customers around the London region for a better perspective on what you will see.
Hope that helps.
