would you advise grabbing a decent seat using BA's online checkin before hand? or just turning up and seeing what they offer
I really wouldn't risk just 'turning up' - after all, a single traveller is an ideal way to fill up all those middle, non-aisle seats without splitting up families
Definatley check in online 12 hours before the flight - you'll probably find you already have a seat allocated, which may be perfectly acceptable. If not, simply chose a new one. Be aware though that this allocation isn't set in concrete - if they decide to move you later, that's just unfortunate.
Just in case you're not familiar, the online checkin system is very easy - but make sure you have your 6 letter locator code to hand as well as the flight details. You get a seat map with a picture of where you're currently allocated, and just click on which ever empty seat takes your fancy. On a longhaul, I'd personally recommend an aisle seat rather than window, but thats personal choice.