These days you can eat out for very little (think pub chains which offer a meal and drink for £5 or 2 mains for less than £10) and you can pay hundreds, with everything in-between.
Some people are buying purely on price....so cheap is imperative and quality not really an issue. For others, quality is everything and price not an issue. For most people, they would like something which seems pretty decent, and for a treat meal, more than decent and possibly luxurious......once you're into this area, you have to expect to pay, but the question becomes about value and being ripped off too. No-one wants to feel they had mediocre quality and service, but were charged extortionate prices.
The problem is that some people have no idea what is a reasonable price to pay for fine dining - they seem to think it should be available for £10 like a chain restaurant meal. Because a chain pub includes a drink and 2 side dishes in a fixed price menu, they feel resentful at being asked to pay extra for side dishes.
If the prices seem to be for fine dining, but the experience and quality aren't up to scratch, it could be worth emailing the pub to let them know your disappointment, or leaving a review on trip advisor or similar. Feedback is useful and resturants would rather hear your niggles than you just tell everyone else about them. Of course, you may find they disagree or you get no joy, but I always think it's worth telling them if you're disappointed. Perhaps mention it was a treat - I don't think you can complain about the prices - presumably you saw a menu before you ordered - but if you think the quality or service didn't warrant the prices, you can say that.
If however, the restaurant just charged higher prices than you like to pay (and it does sound a bit like that) then I don't think you can complain - no-one made you go there, but you went knowing the cost and later felt aggrieved by it.
With meals, I find it's best to have a rough idea of likely price before going out. You need to write off the cost - because if you choose to eat out, it's unavoidable. The question then is did you enjoy the meal and perhaps if you felt it was good value - that's not cheap, but worth the price you paid.