We had a mothercare one which was quite similar to the nipper. While the big wheels made it reasonably easy to push with a bigger child we found the shape of the footrest didn't work as he got bigger. Both are quite flat, so the childs legs fairly straight out I front. As he grew hed wither end up with one leg dangling either side or trying to cross his legs to get some support. He needed the buggy due to a hip condition which made either position uncomfortable quite quickly.
We tried a friends mountain buggy which worked a bit better for him, although at 5 he was still having to sit with his knees bent to get any support.
Our solution was a bike trailer, many have kits to use them as a buggy but actually I found towing him behind the bike worked well and avoided him needing a buggy I school which he was quite self conscious about. That lasted till 7 or so.
We did also look into some of the SN buggys, but yes, prices are a bit eyewatering.
Special Tomato jogger was on the lower end of the scale and the couple of people I know with then think they are great. Again would probably last till 7 8ish.
The older mountain buggy was bigger in the seat than the current ones, and they also used to do a special needs one with a longer foot space. Worth watching Ebay as they did come up sometimes.
I may well be a bit out of date now, its been a few years since we needed the buggy, but the right one makes so many more things possible.