First of all you need to look at the right stuff for your boiler type and your water pressure.
Go to a Plumb Centre, or a similar plumbing and bathroom merchants. They will be much better than places like b&Q and Wickes, or Bathstore.
PC will have a huge range of stuff from basic to very very high end. The choice can be bewildering and it's easy to get bogged down so I'd say from what you've said about budget look at Ideal Standard for sanitary ware, it's very good quality without being flashy/gimmicky or stupidly expensive. The staff in there will sit down and go through the planning with you if you give them accurate measurements and they are not pushy salespeople, they just want to help you order the right stuff.
Steel enamel baths are the best quality but really expensive for a good one (the cheap ones are thin and the enamel chip easily) and they make the water go cold quickly. Don't get a really flimsy thin plastic bath either, they are awful, spend out on a better quality one. Spend most on the bath that will get the heaviest usage, and least on the occasional one. Likewise bath panels, cheap flimsy plastic ones look awful and crack/come off easily.
Are you going to have shower cubicles? If you are having shower trays I'd go for the very solid composite material ones rather than plastic. I think if you can get a bathroom fitter to put up a partition panel and and build a recess with a lip/step and tile the whole thing out like you get in hotels/changing rooms it will be cheaper and less trouble in the long run than buying expensive glass cubicles/doors and trays, but i think some fitters are reluctant to do this as there is a greater risk of leakage.
Avoid Triton showers like the plague, look at Mira, Aqualisa, Grohe. Most plumbers would choose those themselves. Also some digital showers are a bit to clever for their own good and there is a lot to go wrong on them. Better to stick to good quality but simple
If you live somewhere with really good water pressure you can get away with a gravity shower or a good quality shower mixer kit over a bath. If the water pressure is not great you are better off with an electric shower or a pumped power shower. But you really need to get advice on what you can and can't have from a plumber first, as I said, depending on your boiler type etc.
Taps, be really careful. Cheap taps will be a pain in the neck and fall apart really quickly and you'll have to take half the bathroom apart to replace them. Bristan are good.
Tiles you can really save money on. Again the choice is bewildering but it's not difficult to tile fairly cheaply and I have found B&Q to have the best range of sensibly priced tiles.
Go for underfloor heating in the bathroom that gets used the most. Have heated towel rails but make sure you get ones that are big enough - it's annoying to not have enough space to hang/dry towels.