In my experience BA are fairly strict but will normally allow a few Kgs over. Unfortunately I think 30 and above they will definitely charge you for. I have just flown to BKK with 28 kgs and was not charged.

Once when leaving from Gatwick they were charging everyone for every kg over there allowance. They even weighted your hand luggage so that you couldn't just moe things into that if you were a couple of Kgs over. Luckily I was trevelling light that trip but it was a very slow checkin as everyone argued their excess charge.