Go along, NEVER quote a commercail without seeing it.
Use a tape measure, the ultrasonic are not reliable and don't get too excited about either, no one cares how you measure its your price and cleaning results that matter.
i would not bother with shirt and tie, in fact some people don't like it, turn up as you would for cleaning the job. Don't offer risk assessments, pat results, method statements or anything else unless asked for, then say yes I can do that and then ask how on here, if you don't know the answer.
I always use HWE, but they are the odd job you can not, but not many. if you have a porty and no LM system then don't offer it, again no one cares its the results that matter, as long as your HWE does not cause any problems.
(and if they is air conditioning it does not cause any moisture problems, and if there is not open the windows).
Work alone and do one floor at a time, maybe one friday evening, one saturday and the othe the following weekend, or get someone to load/unload buckets.
As for price thats up to you. I would work out how much a whole floor would be at £x's per metre and if that sounds like a good days work, stick with it.