You can always ask if the girl is prepared to quote you a 45 minute rate, the worst that can happen is that she tells you to piss off.
Why would they do that?
I can maybe understand in a parlour, but not an indie.
I saw a wg in her own apartment who was, in my opinion, just a bit too expensive for the hour @ £170 but a good punt all the same. I asked her if I could book her again for a 45 minute session @ £140 (still expensive, but I wanted to see her again - her half hour rate was £110 so I split the difference). She just said "No. Half hour or full hour only".
It doesn't make sense to me, so I now spend my money elsewhere