We opted for uPVC frames from EuroTech Windows
. We're very happy with both the windows and the installation.
We've kept our old awnings - double glazing helps but direct sun will still warm the room up nicely with the regular glass option.
Is it worthwhile? Depends on so many things...
Financially, you save on heating & cooling bills (how much, all depends), ours have locks so we save on insurance costs, and painting costs. In theory it adds to the value of the home, but I don't know how much.
Will it offset the cost of installation? It's an open question, I think you'd have to live in the house a long time before it did (at current energy prices at least, that may change).
In our case we needed to replace at least 2 windows anyway, and the rest needed painting, so one way or another we were going to be spending money on them. The old window screens also didn't work very well at keeping the bugs out.
But there are also less tangible benefits.
The drafts are gone, so in cooler weather at 20C the lounge *feels* warmer than it did at 20C previously. And it heats up much faster, which is convenient. Presumably also true in reverse for cooling in hot weather but we haven't had a chance to test yet.
The old windows were awkward to use and couldn't open wide, so we couldn't easily cool the house down at the end of a hot spell. The new sliding windows are great (we wanted tilt&turn, but they didn't work with the layout), and we now have insect screens on the french doors so we can leave those open too.
For us, having more control over the environment inside the house and being comfortable is worth the price.
From EuroTech, 45 panes of glass (including 4 sets of french doors), fully installed with insect screens came in at a little under 40k. Nearly 3k of that is the retractable screens on the french doors.