That is fairly important,they cost to deliver,Plenty of window manufactures in and around london.
Some supply only companies will send out a technical person to measure and select the correct products that your installer can fit and your installer can register with fensa.
I haven't used them before but thought they had a good website when I was looking into prices etc.....
Most UPVC manufacturers will buy stock extrusions from X Y Z and fabricate the windows to suite your given sizes.
Wickes sell UPVC windows if you were not looking for a specific size and were building from scratch so could accommodate whatever size or thereabouts that you liked the look of. But as you are retrospectively fitting I doubt this will be of use unless your windows are the stock sizes.
I know a place in Romford which I would recommend having used them before if thats no too far for you, they're called window tech trade.
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