Does £1 per house seem reasonable or is that a bit excessive?
That's the basis for giving a decent quote, but there's much more to it than that. It really depends on the job.
For new work on standard residential properties, I always count the windows and doors (which we clean) and charge £1 per window; then round up and maybe add some more if we don't really want the job; or if there's some awkward access areas; or if the customer appears to be a bit 'demanding'.
There's no formula to pricing; it's a bit of an art; I honestly think it all boils down to how much do you want the work? If you don't really want it, price high, if you want it price reasonalbly; never price cheaply 'cos you'll end up giving a shoddy service and eventually dropping the job.
Round my way, there's lads getting (probably) £20 for the same sort of properties I'm cleaning for £10, and there's (probably) others who're doing similar properties for £5.
I bet it's the same all over the UK.
PS. I'm from North Shields, but live and work in South East Wales: I see you're a fellow Geordie!