If it's the coffee it's more likely to be the **** and additives they bulk it out with. Always drink low to med roast quality coffee and you should be fine. Most coffee in the UK is so weak and low in caffeine content it's unlikely to be that.
And remember 'strong dark roasts' are lower in caffeine than mild roasts - over- roasting destroys both caffine and the flavours in a good coffee.
Am currently in Costa Rica where they know a bit about good coffee, and this is their coffee experts views too.