There is no parking at Waterside unless you work there and even so that is extremely limited. Unfortunately there is basically no parking near Waterside apart from a carpark on a road nearby(tarmac way) but it becomes completely congested in the morning, its not actually anything to do with BA but first comes first served(nature reserve parking?). I wouldn't rely on parking here.
The best bet is to basically park at short term airport parking and get a bus, or get a lift..
Others might have better ideas but its not ideal, Waterside is very close to the airport itself, so as you can imagine, parking is very limited and at a premium!