A lot depends on what you want. Kind of like how long is a piece of string? If you think that you want nothing moved then break it down by jobs. Of course things can be cheaper if your builder can do the lot and doesn’t need a seperate plasterer. Also if they don’t have a lot of work they may price it lower.
Demo of a small-medium bathroom- 1 days labour + skip for disposal (in London that’s anything from £150-£250/day+£250/skip). If you only want a small amount of re-wiring or moving of pipes they may be able to fit this in the same day. If you’re talking about a whole new lighting scheme or they need to rip up the floor to re-route pipes you will probably need another day.
Re-skim if it’s a small room maybe your guy can do it the same day he demos but probably another days work. Materials cost is negligible (£20-£30). Leave that 2-3 days to dry.
Tiling & grouting is another days work, +tiles (if you use standard metro tiles 200x100 you can get them ~£10/sqm. Fancy tiles can cost a LOT more- remember to allow 10% wastage), and can probably also lay flooring as well same day.
Then 1 day to site & fit bathroom furniture as well as paint and finish. Materials can cost anything from about £600 for a standard suite to silly money. If you price match and look for discounts or your builder gets good trade discounts you can get a nice bathroom for ~£1000-£1200
So- 4-5 days work + 2-3 days in between for plaster to dry. I’d say in London £750-1250+VAT is a pretty fair guess for labour. Materials can cost a lot or a little but for a moderately good standard of finish I’d say at a minimum you’ll spend £500-600 on bathroom furniture and £200 on tiles, +another £400 on incidentals (skip, new switches, pipes, wiring, plaster, lights, etc.)
So for a small-medium bathroom I’d say you’ll spend £2-3k for a refit. Potentially more if it’s bigger, or you buy nicer fittings/tiles. This is based on my experience doing two bathrooms in London in the last 6 months. If you have a bigger bathroom (say 5msq or larger) then the costs will increase (potentially a second skip, or more than 1 day for demolition, more tiles, etc etc.). For a large bathroom I’d guess that it’ll come in around the £4-£5k mark.