Hughs comments are correct, but to be more specific, it depends on the number of people in the house, useage patterns, and water pressure. I've just changed the incoming water mains, and the water pressure is now great, so cold water can be drawn off anywhere without affecting the hot supply from the combi to the type of shower you're thinking about, but as Hugh says, if someone else turns on the hot tap, then the shower goes cold.
But the real question is, what are the plumbers supposed to be doing for the £850. They should have told you what type of shower they are going to fit within the price, so are they supplying it in their quote, alongside the shower tray that they are obviously fitting. I think there's a bit of miscommunication going on here.
With an adequate cold supply, and a thermostatic electric shower, the shower pressure would drop but still stay hot, but you'll add another £300 or so to put in an electric shower, as it needs to wired back in to the fuse board. Is the light switch in a convenient position to cut it through the wall to the corridor, or would the electrician need to get in to the loft space, and route it several feet away. You'll need to get someone in to give you a quote.
So what are you doing about ventilation in the bathroom whilst it's being refurbed. If the electrician is moving the light switch, get him to fit a fan with an overrun timer as well.
