If you take the 777 and don't like it, you can always leave asap - I don't think you are bonded in BA, are you? You can then go charter and be only a year or two behind where you otherwise would have been.
If you go charter and turn down the 777, BA may never invite you again and you will always wonder what it would have been like. Remember, it's always what you DON'T do that you regret.
I've never flown it, but I have a feeling that the 777 is probably THE ultimate big jet and you'll probably love it. But the sectors and the lifestyle? No thanks.
I would never have forgiven myself if I had missed out on my B767/A330/longhaul phase, it was great fun, but longterm give me charter and shorthaul.
Shorthaul is for pilots who love flying, longhaul is for pilots who love being pilots.