There is a very good reason, with there being no official open it is just when the first bank puts the first price through, but what we have found is that first thing on a Monday morning liquidity is pretty rubbish to be honest which can result in wider spreads coming from the banks. If we opened up earlier, we may see some really strange spreads and price spikes which could result in trades being done that clients may not have wanted etc.
Personally I have previously been an FX dealer and whilst working the night shifts in London, we used to open our market at 10pm Sunday night (7am Monday) and the liquidity was truly shocking.
Technically we could set things up so that it started at any time we want, but imagine if we started at 6am, and there were misticks all over the place... it would not be pleasant!
I understand the reasoning from that perspective but when you are in a trade which is in profit and doing nicely on Friday night but then you wake up on saturday morning and find your still in profit but the market is poised to go against you on open on Monday morning and there's nothing you can do about it as the platform does not let you amend stops or take profit orders over the weekend, and then Go doesn't open till 8am by which time 30+ pips of your profit is gone then that doesn't leave you very happy.
Also my strategy provides quite a few entries on monday mornings so i was missing those.
A good solution would be to allow stops and TP orders to be ammended on the weekends.