The reason why the ISPA did not like DoH is because UK law said they had to block/filter traffic. Per court order, a UK ISP would be obliged to block things, and if DNS was no longer an option, they would potentially start having to do IP blocking and BGP sink holing (PDF warning):
While I'm sure the ISP techs may have had some misgivings about DoH (plenty of tech-mind folks like Paul Vixie do), the strong response from the ISPA may have been guided by the lawyers.
* https://www.icann.org/sites/default/files/packages/ids-2019/...
That law now seems to be dead, which may reduce their worries on the matter:
* https://arstechnica.com/tech-policy/2019/10/uk-government-ab...
While I'm sure the ISP techs may have had some misgivings about DoH (plenty of tech-mind folks like Paul Vixie do), the strong response from the ISPA may have been guided by the lawyers.