I think you ought to price what the job is worth to you. If you need the work, go cheap; you'll be to cut some corners to make it profitable if need be, and if you don't need/want the work go high.
We normally lose out - I'm sure it's just on price - when it comes to quoting for stuff where other window cleaners are asked to quote too; and believe me, we try our best to sell ourselves; I even ask Wor Lass to stick a bit of 'slap' on!

It looks a straightforward job though, especially if you're van-mounted (it'd be to big for me and my backpack); no exterior access issues.
Inside could be another matter though; climbing over desks and stuff, but they're simple one-pane windows to squeegy.
Personally, I'd err on the side of 'cheap' rather than 'expensive', 'cos there'll be other window cleaners who're asked to quote, who'll go in cheap, and then do a 'cheap job'; but everyone will be happy. The company gets it's windows cleaned, (they won't be looking for perfection) and the window cleaner will be able to turn-over a good amount too; with only one-point-of-contact for payment; though you'll probably have to wait a month (at least) to get paid.
I wouldn't go lower than £200 though.