Double Glazing Salesmen

I have recently needed to install new glazing throughout the house. I thought this would be a simple operation. I would telephone various local and national companies, invite them to cost the job, provide appropriate literature of their product and then spend a weekend to decide which one to go for.
In reality it is quite different and I hope to warn anyone else if they are considering new glazing. I will not name names but the majority of double glazing salesmen I found pushy, rude and intimidating. Out of 6 salesmen, all apart from one insisted on delivering an hours long sales pitch complete with the same old window sample and each one criticizing every other company, supplying derogatory comments on the quality of their products and their service. This sales pitch is fruitless because you tend to just glaze over and nod your head. All apart from two needed details of nearest schools, neighbourhood watch schemes, shops etc which seemed more like a casing of the joint than conducting a quotation. All salesmen apart from one insisted that their quote was only available for that date and unless you signed the form they could not repeat it and would charge you a far higher price. One company actually gave me a lecture of how infra red light changes its wavelength once its in the house so that it cannot escape again. I did not let on that I have a degree in applied physics, and just chuckled to myself afterwards.
One company would NOT leave until I had signed and handed over a post dated cheque which I then had to reject the next day in the 7-day cooling off period.
Having said this one company did not bring in a window sample and did not go into detailed manufacturing techniques but left a brochure as requested. This person kept the sales pitch short(er) and gave more information about Health and Safety regulations which need to be adhered to - which seemed a lot more appropriate since other companies did not agree on what the health and saftey regulations stated and if I had taken 3 of them I would have ended up with extra expense because of their wrong interpretation of the regulations.
In general I was disgusted at the way these companies conducted their sales techniques and feel there really should be some kind of regulation introduced into the industry. The potential customer should NOT have to sign papers and hand over cheques to get salesmen to leave. I asked for a quote and details of which I could sit down and compare at a later date. Only one company came close to this. Only one company did not have a price that would not stand for more than a day.
There was one method I did find, quite by accident, that seemed to work effectively in reducing the time and annoyance caused by the salesman. This was is to book appointments too close together. The sales pitch generally lasted two hours each but if you booked appointments at hourly intervals then the first salesman had to cut short his lengthy discourse and didnt engage you in the "you must sign in order to get this offer".
I did find one internet site that offered an on-line quote - you filled in a form with the size and style of each window and door, hit the button and it returned a quote. A couple of the salesmen could not believe this could be done. I do not understand this viewpoint. Every other industry can offer an online quotation. The salesmen only roughly measure each opening. They then refer to their own pricing books to come up with a quote so why cant this be done by computerised systems?
Overall pretty totally disgusted. At the moment I am thinking of taking the quote of the company with the least pressure. They were not the cheapest, neither the most expensive but the left literature, they were the most polite and did not ask me to sign any papers there and then. They are a small local business. So this will be one-up on the nationals.