In order to request a breath test they would have to suspect that you had alcohol in your body, had committed a moving traffic offence or had been involved in an accident.
You hadn't committed and offence, driving too slowly is not an offence in itself although it could be classed as driving without reasonable consideration for other road users, and you hadn't been involved in an accident. That just leaves suspicion that you had alcohol in your body, if they can't smell it on your breath and they believed you when you said that you hadn't had a drink, there is no power to demand a breath test.
So, don't get annoyed, they must have believed what you said.............obviously don't know you do they ;D ;D ;D only joking ;)