I'm not sure what you mean regarding overweight = 'SP likely to fall'. Overweight, under-weight, equal-weight, market perform etc. are broker recommendations rather than words you typically see in an RNS. Broker recommendations can shift a share price but when you read their notes they have very diverse interpretations of the terms that they use. It's not atypical to see an underweight recommendation on a company with a price target 10-15% higher than the current price. How can they recommend clients be underweight and yet think there's a 10% profit to be made? The only real answer is that the brokers are often inconsistent.
As far as RNS messages are concerned, I understand that big trading outfits 'parse' RNS messages for keywords and so their automated trading systems 'know' whether an RNS is good or bad almost as soon as it hits the wires. I read somewhere that some of these systems trade on the information. I think traders learn the codewords in RNS messages so a quick scan can tell you whether things are really good or bad in-between the positive spin.