My decorator charges £200 a day plus materials. She has painted my entire house now (in stages), and some rooms have taken longer (so cost more) because of the prep required. Filling in cracks, taking off old wallpaper etc is time consuming, as is lining walls before painting. Where we were happy with paint onto plaster (mainly upstairs) it took far less time than where we’d had wallpaper (never again!) in reception rooms, so the charge varied. We talked all of this through before she started each job, and in theory the lines walls will just need painting over in future.
That doesn’t help, OP, but the cost will depend on things like the state of your walls (not always obvious if they are papered). Be sure to get paint the decorator knows is scrubbable, too. I chose one lot (Valspar, I think) that was supposed to be scrubbable, but ended up needing to get it done again because a wipe with a damp cloth left a mark. Now we go for brands the decorator has used in her own home and knows will clean well.
£8000 seems a lot for 2 beds a sitting room and kitchen(?) though. Even with top of the range paint I’d expect it to come in at less than that, unless there are lots of repairs needed.