It's definitely a set piece. It looks like they built about half of one side; pretty much one wheel and the door. Also it's very likely those scenes were filmed in the US whereas the APC was built and subsequently scrapped) in the UK. The cost of shipping it to the US would have been astronomical.
Here's the door handle on the original full size APC:
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Handle.jpg [ 93.34 KiB | Viewed 3202 times ]
This is the one from 'Ride at the Speed of Fright':
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vlcsnap-2021-04-30-18h23m03s934.png [ 485.62 KiB | Viewed 3202 times ]
Close, but no cigar.
Harry