I’m going to take your three questions in a little different order, from the order in which you asked them.
Because the retail forex trading day never ends (except for weekends), there is endless disagreement among traders about what constitutes a “day” for purposes of plotting pivots, identifying daily highs and lows, etc. This disagreement doesn’t occur in exchange-traded markets like the NYSE or the LSE, where an opening bell, and a closing bell, tell everyone when the day has begun, and when it has ended.
Since the beginning and end of the forex trading “day” is a matter of opinion, everything you are about to read is my opinion. If my opinion doesn’t appeal to you, ask around until you find an opinion you like.
In my opinion, it doesn’t matter where you live, or where your broker is headquartered. The forex market is worldwide and continuous, 24/5. It has certain rhythms, which don’t depend on where you, or anyone else, happen to be located.
Well, I don’t follow your logic. If the forex trading “day” was based on the particular pair being traded, in your case the GBP/USD, why would you favor midnight-midnight in the home market of the cross-currency (USD, in this case), versus midnight-midnight in the home market of the base currency (GBP)? Especially considering the fact that twice as much volume is traded in London as in New York?
Let me suggest a different way of identifying the “forex trading day”.
Here is a 60-minute price chart of the GBP/USD, the pair that you like to trade. The chart covers roughly 20 trading days. Below the price chart, there is a tick-volume histogram, showing trading “activity”, as measured by number of price changes per bar.
Does the rhythmic rising and falling of forex “activity” suggest a daily pattern to you?
To me, it suggests this pattern:
The low points in daily forex trading activity typically occur between 5pm and 6pm New York time, day after day. So, I choose this time as the “logical” time to designate the end of one “day” and the start of the next.
In the second chart, above, the green day-separators mark 5pm New York time (EST) each day.
Because the U.K., all of Europe, and North America (which together account for 74% of all forex trading volume worldwide), all switch from standard time to daylight saving time, and vice versa, at roughly the same time in the spring and fall each year, 5pm New York time (10pm London time, and 11pm Zurich time) works year-round.
If you doubt the correlation between tick-volume (charted above) and actual dollar-volume in the forex market, here is some work I did on that subject — http://forums.babypips.com/newbie-island/38202-volume-forex-market-4.html#post246891
As I mentioned, all of the above is my opinion. Do with it what you will.