................. pssst! ........ Annie.......... a word in your ear....... it's SARA without an H.
She's very touchy about the H :o(
Anyway .......... as Jack says........ "piece of string"
One thing though. In this situation, people tend to focus rather too much on fittings. There are high-end, ridiculously expensive ones about, but even good basic units are of pretty good quality these days .......... mostly.
Design is everything. Italian makers like Ceramica are very stylish.......... check out:
http://www.plumbworld.co.uk/
Labour is the biggest cost by far. As mentioned, separate trades (on a quote, not daywork) work out the cheapest, but need co-ordinating.
Next best would be a price for the lot from a small, local firm who have all the tradesmen, or can hire them in as needed.
There's a lot of work here, and probably a few unforeseens that would necessitate alterations. I'd be inclined to budget for the 20k (almost 3k will be VAT). Get involved, and do your homework, you could well be pleasantly surprised.
Good luck