There is (or used to be i left 2 years ago) a really nice Cabin Manager at BA LGW, I'll not make his name public but it's a small base so ask around who commutes from AMS.
I used to have a 4 hour train ride to and from LGW for work. Had a variety of methods of coping with accommodation needs whilst on reserve etc. B&B's specidically aimed at crew are numerous, nearly all the hotels offer BA staff deals (check the internal information) and the Yotel in the south terminal is great for a quick shower and nap if you travel over night (you pay by the hour).
That and great crew at gatters offering a bed for the night (no not in a saucy way).
As for getting called for LHR trips from Standby, unless things have changed massively with regards to dual basing agreements with the Unions, NOT EVER GOING TO HAPPEN.
As for carmen bids, be generic bid for 7 day trips first, then back up with 6 day trips, then 5 and so on, if you try to be too speciic you end up with a roster which can be difficult to commute.
There is company policy regarding commuting on STBY. it used to be a 3 flight buffer but many peeps just use loco's which can have a similar price to stby if you book in advance and are confirmed.
Have you been at Gatwick long? it seems as if this is stuff you would pick up from other crew if you'd been there a while; it's just getting 75% as a Junior is by no means guaranteed.