as far as I can make out, bathroom showrooms charge higher prices than plumbers merchants.
You can get a white bits anywhere, and they pretty well match.
Modern WCs have low water content and often don't flush well, which is a source of much sadness. I'd go for an established British make such as Twyfords. I got a fairly rare one recently from QS at a good price. Most have a Horizontal outlet. If your soil pipe goes vertically through the floor, you need to choose one where wall-to-pipe-centre distance matches, which may be hard.
Unless you have a novelty-shaped arse, don't buy a novelty-shaped WC. Square and rectangular seats to fit are incredibly expensive, if not unobtainable in a few years.
Please don't box or tile it in so it can't be maintained without smashing something. Cisterns with a button on the lid are often difficult to maintain. Ask the salesman to take the lid off and time him.
Look at the thickness of the bath. They are mostly acrylic these days. Cheap ones are thin. Better ones are "reinforced," though a Carronite one will be outside your budget. Think of a thin one as disposable.
If your bath comes with a flimsy plastic front panel, save time by throwing it straight in the skip, rather than doing it in a months time after it has cracked from your knees pressing against it when you swab the bath. A painted chipboard one might cost about £100. The panel must be retained by a couple of screws so it can be swiftly removed to tighten the waste or to dry out spillages. Don't allow anyone to glue it into place with silicone or tap it with tiles.
Don't get a bath with the taps or waste in the side against the wall. How will you reach them without removing the bath or knocking a hole through the wall? A simple rectangular bath is cheapest to buy and easiest to fit. Examples
Don't put a cabinet or shelf above the basin, because sooner or later a glass jar or bottle will fall out and crack it.
For taps and accessories, nothing is as durable as chrome.