It looks to me like the scrutiny of further education provision brought about by the Robbins Report led to H-W being awarded University status. This would only have been done if their level of teaching and assessment was equivalent to the classic university. What we think of as the classic "Polytechnic" was a creation of the local education authorities's responses to the Robbins Report. This new drive in vocational learning, to match the new "red brick" universities, led to the take over of existing, willing, educational colleges, schools, polytechnics (of which there were scarce few calling themselves that prior to the 60s) etc. The government funding injections into technical subjects following WW2 (they got very keen on training the youth in engineering and electronics following that little conflict for some reason!) were no doubt dependent upon submitting to LEA control under Wilson and Crosland etc. Where provision didn't exist to expand, LEAs created it de novo.
H-W doesn't fit any of this. It was a "private" establishment long before this time, and I expect it was "boosted" to university status in order to distinguish it from the slightly more vocational, work-place based, training envisaged for the polytechnics, i.e. the skilled factory floor engineer rather than the design department boffin types.
Or you could see Crosland's reforms as being the formalisation, standardisation and generalisation of the apprenticeship system that operated prior, which took youths from ages 14 to 18 and delivered a very specific skill set. As the school leaving age was raised, the foundations taught during an apprenticeship, such as more advanced levels of numeracy and literacy, had to be transferred entirely to a schoolroom setting. This led to what you might call assembly-line learning.
TL;DR, H-W never called itself a Polytechnic and was a university before the widespread emergence of Polytechnics that were modelled on the few places prior to that era which already called themselves that.