Hi Flossy. Notonething and Coughle are right. YOU are the customer. And YOU are always right.
I'm a tradesman. The view I take is that it's my job to leave the customer happy, and try to foresee any problems. Otherwise, what's the point in employing me?
What I think your plumber should have done was consider where your choice of tap would fit on your bath. It sounds like he's not done this, realised too late that there's only one spot they'd fit in, and thought "shit, she won't like this, I'd better open the packet so I'm not down a couple of hundred...". You don't have to be a rocket scientist to realise that 9/10 people are not going to want to step over their taps to get into a bath.
What he should have done, upon realising, was tell you straight away then discuss how to go forward. He may have been thinking about time taken to order new taps, he has other jobs to do, best to get this one wrapped up, etc. I'm speculating, but that's my guess.
Remember, you haven't bought the taps. He has. And he shouldn't be invoicing you until the job is finished and you're happy with it. Whatever you do, do not hand over any money until both these conditions are met. Money is leverage with tradesmen.
Plumbers make a lot of money, and I might argue rightly so, as it takes time to train and is a highly skilled job. But if you feel that skill isn't being exercised, you need to be business-like about it. I have lots of customers I like, and get on well with, but at the end of the day I have to remind myself they are just that. They aren't my friends, although we may have a friendly relationship. They're paying me to do a job, and that's exactly where your leverage comes from.
To be blunt, the one thing a tradesman needs to be thinking is, "what can go wrong with this job, and how can I prevent that?". From your OP, it does sound like he's not done this. And your question is, "Am I not paying you to foresee these problems?".