Progress, I think. When shooting screenshots of everything for Airalo support, I went to Settings / Network and Internet / SIM cards and found down that page Preferred SIM for Mobile data was set to 3 UK (my "normal" SIM), and had not been set to "Play", despite the installation of the eSIM having gone OK, data roaming and mobile data switched off for 3 UK and data roaming and mobile data switched on for "Play".
Once the Preferred SIM had been set to "Play", browsers worked, WhatsApp blipped so I think we're there now. Not the easiest install but Airalo have been supportive throughout.