Newcastle don't even look at your GCSEs and think Imperial don't now either. Some unis e.g Cardiff, Oxford are obsessed by GCSEs. Each uni has VERY SPECIFIC ways they process your application, so do you research very carefully.
ALL unis will want 3 As at A LEVEL, with some wanting an A* too.
Lower GCSEs mean you need a higher UKCAT and/or BMAT to compensate.
So, in other words, less impressive GCSEs aren't the end of the world but it just reduces options of where to apply.