I bought a serious renovation property 3 years ago (stillon-going) & if your having all the windows replaced (as I did) or a significant number of them, it’s not worth the hassle of sourcing the windows & fitting them yourself.
If you DIY & have not done it before you have to go through the learning curve - it’s not difficult but 2 windows a day! Then you’re stuck with notifying LABC, paying for the inspection to get a certificate of compliance (which you need) & you wont get the benefit of an independent 10 year warranty. Fitting charges for a competitive, local company will be £50 - £80 per unit (windows/ French doors); they can get good/top quality units (I have Rahau) far cheaper than you will have any chance of getting them yourself & if there’s a problem, they will come back & fix it! Personally I would never use any of the large nationally advertised companies (except as a guide); very few companies make their own windows anymore & you will probably end up getting the same units that the established, local boys use but at a much higher price; but possibly installed by dodgy, self employed fitters who don’t give a flying fig about your installation cos it won’t be their problem!
Get lots of quotes (I had 11 which I whittled down to 3 without too much trouble!), look carefully at the product & trust your gut instinct regarding the supplier (carp, waffle etc.). My contract included a load of other work which I wanted to let to a single contractor; the total for the whole project was a lot of money but I can honestly say that my experience with the local guys was excellent; the cost was competitive &, overall (apart from the odd row), the experience was very good. I have posted on another thread regarding budget prices so have a look & see what you think but, IMHO, DIY on anything other than one or two windows is a waste of time & probably money! Oh & give them as little cash up front as you can get away with; 10% if you can but no more than 20%.