I certainly don't see a fall in demand for function work. If anything I've seen a rise and more and more purpose built venues (big farm barn conversions, out of town hotels etc etc and i've been in probably 30 tipis this summer). I'm out 1-2 nights every week and have 60+ already booked for 2019. All paying between 150 & 200 a night depending on distance.
Pub work however seems to be a totally different kettle of fish tho. Lots of venues closing, poor punter turn out due to drink prices and a market flooded with classic rock bands prepared to play for "the love of it" and lots of landlords not prepared to take a risk on different bands / styles.
To answer the op I'd probably say look for somewhere between £50 - £70 for a pub gig and £150 - £250 for function work.