There is no way the agency could know that someone clearly moving out was ending a tenancy or selling. To all intents and purposes the effect is the same, and the standard required by the agency to keep up their reputation is probably important to them. It doesn't matter that the OP would have been happy with a less than stellar job.
I don't think it's fair to have to be pushed into an end of tenancy clean when that's not want I wanted or needed.
We've had one off cleaners in the past when our regular cleaner was away who haven't been able to clean the whole house in the allotted time, but always did what they could
If you wanted a bespoke service then you should have hired an agency that offered that. You are not being fair to a company that offers a standardised service when you ask for a finished result that is less than the one their reputation is built on. The buyers of your house might well badmouth the state of the fridge, the kitchen cupboards, or the downstairs and neighbours may well remember the name of the cleaning company that was parked outside.
If you badmouth this company on the basis that you essentially baited and switched and they didn't fall for it, you won't come out looking good, their headless hen response notwithstanding.
SinkGirl, it's not that moving out/ending a tenancy is a 'special occasion' like a wedding where a premium is added to every item or service 'just because'... End of tenancy cleaning is often a matter of dealing with mountains of left behind filth, fridges full of rotting food, cupboards with all sorts of nasty surprises in them, toilets that haven't been cleaned in years, bathroom floors that have absorbed gallons of pee, mess left behind by pets, ovens and hobs that are completely crusted over with baked on grease, bathtubs that are too dirty to wash a dog in, and lots more.