d1ddled - When you have measured flow with your bucket, we will know how much water is available. To run a bath, you need a lot, if your water pipe can deliver that much, and if you are going to have a combi, it is worth buying quite a powerful boiler. If your water flow is very poor, e.g. you live at the end of a country lane, or have a narrow and squashed old lead pipe, it can be improved by laying a new pipe out to the mains in the pavement.
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for the WC cisterns, I expect they used to be fed from a cold tank in the loft, at low pressure. Now they will be fed straight off the water main. They probably have low-pressure ballcocks inside. You can buy high pressure ones, though I would go further and get modern "silent" filling valves. Whatever the pressure, they are always quieter than ballcocks, and much less prone to chatter or hammer. A plumber can fit them, you don't need to pay heating engineer rates. They look quite different with a very small float. They do cost a few pounds more, but ballcocks are not expensive. it is possible to take apart your old ballcocks and fit a high-pressure cone, but I think not worth the effort, unless you like tinkering.
Here are some examples. They are mostly made by Fluidmaster now, which amalgamated with the company that used to make the Torbeck valve. Those are the two best known brands. You might consider having new flushing valves or flappers fitted at the same time, which give a powerful and reasonably quiet flush. They look very different from the big old syphons. Ask around for a recommended plumber. Unlesss he is very traditional, he will be familiar with them. I don't know if the latest model comes with a pressure reducer; ask him to fit it if it does. It is just a bit of plastic in the pack.
Because your pipes are all now running at high presure, any looseness or inadequate fixing will cause more noise. Certainly have all visible pipes clipped tightly to the wall for a start. Maybe you will not need any underfloor pipes clipped as well. Old plumbers are usually very experienced with noisy pipes.
If the taps are noisy, or start to leak, consider buying British taps, which have larger waterways than most imports and run quieter. Bristan is a very good brand. If the taps or other fittings are old and worn, they are more likely to be troublesome at high pressure.